Cygnar is already a faction which enjoys supporting mercenaries, as most of their spells and feats will affect friendly models anyway (like Arcane Shield). What's cool about this is that it makes sense according to their fluff! There are, however, some warcasters who have spells or feats that only affect friendly models, and those are the ones which matter most when it comes time to decide whether you want to take Murdoch. I looked through the book to find any warcaster with a feat, spell, or ability that affects only friendly faction models, and this is what I came up with:
pHaley - Feat: Blitz - The rest of pHaley's spells and abilities don't rely on their targets being friendly faction models, but her feat only affects faction models, allowing them to make an extra attack.
eHaley - Temporal Acceleration - Much like her prime version, eHaley's abilities don't rely on faction models, but much like her prime version's feat, Temporal Acceleration affects only friendly faction models, and an extra attack is always useful - although this only affects a single model, so it's hardly amazing.
Kraye - Easy Rider - While Kraye is generally regarded as running with a lot of warjacks, it's useful to know that he can grant Pathfinder to all friendly faction models for one turn, meaning that you can reliably get that charge that you need with your Mercenary unit.
pNemo - Deflection - Hardly game-breaking, but pNemo's Deflection provides some extra protection for friendly faction models against ranged and magic damage rolls. While this might not do much for Nyss Hunters, adding extra armor for an Ogrun Assault Corps. would make them much more resilient against enemy fire.
Sloan - Feat: Firing Squad, Refuge, Return Fire - While Refuge and Return Fire will be less effective with Mercenary units as they only affect a single model, Sloan's Feat will be fairly significant for either a unit of Nyss Hunters or Ogrun Assault Corps. as boosted attack rolls can allow them to reliably decimate their intended targets with their shooting attacks. Note that this also affects their Assault attacks!
pStryker - Feat: Invincibility - Stryker is not terribly popular among the Cygnar warcasters, often overshadowed by his epic version, but he has a lot of great toolbox spells that affect friendly models, but they were often left in the dust when he popped his feat. A solid +5 ARM will be welcome by any mercenary unit now, and keeps Nyss safe from blasts for an extra round or makes the Ogrun Assault Corps. even more survivable (sense a trend yet?).
eStryker - Feat: Rolling Thunder, Deflection - While Deflection will affect Mercenaries the same way it does with pNemo above, Stryker's feat also significantly affects friendly faction models. Being able to advance and make an extra attack is pretty significant considering Nyss Hunters are Weapons Masters. Combined with Murdoch's Assault (Order), this means that you can really clear out swaths of infantry on eStryker's feat turn and either roll into the second line, or have another chance at killing models if you failed the first time (like against Gatormen).
Blaize - Feat: Divine Intervention, Crusader's Call, Transference - Blaize probably benefits better than most from having Mercenaries count as faction models. Not only is she capable of claiming their souls for her feat, but when she gains such a volume of souls from her feat, the remaining survivors can even use the extra focus points with Transference. Crusader's Call is just the icing on the cake here, especially since the mercenary unit can Assault. Imagine what a unit of Nyss Hunters could do with an extra 2" charge range, shooting their bows, and being able to boost at will!
Khador
Khador actually has quite the broad selection of spells and abilities that affect only friendly faction models/units. While the ever-present Iron Flesh can already affect mercenary units, there are a slew of other abilities that Khadorian warcasters have that only affect faction models. With the new MUA, I think there are some very interesting considerations that can now be made with said units.
pIrusk - Feat: Undying Loyalty, Battle Lust - pIrusk offers a lot to faction models, allowing them to pack quite the punch. Here, I think Halberdiers win out the best since they're cheap and can hit their targets fairly reliably thanks to Powerful Charge, meaning that Battle Lust will allow them to apply a lot of damage, especially during his feat, where they can also become a wonderful tarpit for the low cost of only 8 points with Valachev. That said, however, almost any unit can benefit from his feat, save except for Boomhowler & Co. where it's a bit redundant. Croe's Cutthroats also would appear to be a great option here as well.
eIrusk - Feat: Desperate Ground, Martial Discipline, Total Obedience, Battle Lust, Fire for Effect - Supreme Kommandant Irusk is probably the best warcaster to run with faction mercenaries, period. Much like his prime version, he offers a LOT to Halberdiers and they turn into a really cheap tarpit while your Winter Guard Death Star sits back nice and comfy, waiting to clear out the foe with rocketeers or boosted blunderbusses. Irusk would also make Nyss Hunters scary between Battle Lust or Fire For Effect on top of everything else to make them well worth their hefty investment, but they would likely be best sitting back as a second wave to follow up your Winter Guard. In any case, eIrusk should absolutely consider taking Valachev and a mercenary unit, as he has more to offer them than any other warcaster - Khadorian or otherwise!
eSorscha - Shatter Storm - eSorcha loves her Winter Guard probably more than most, and her feat also works well with mercenaries anyway. Since Valachev already has some great synergy with ranged units thanks to Zephyr, Shatter Storm would just be icing on the cake for a unit like the Nyss Hunters, but I think there could also be a special shout-out for Croe's Cutthroats, since poison will practically guarantee a Shatter Storm trigger, even against higher ARM targets like shield-walled infantry.
Strakhov - Feat: Iron Fist, Sentry - While Sentry won't find many uses since it only affects one model, any mercenary unit with melee attacks would love to take advantage of such an awesome feat! With Zephyr on top of that, you can make sure that everyone can get your intended charge target from across the board. Steelhead Halberdiers could take advantage of this feat probably the best for their cost, as it extends their threat ranged to "OMG" levels.
pVlad - Signs & Portents - Everyone loves (or hates) Signs & Portents and with it affecting faction models, mercenaries can benefit from such an awesome spell too, thanks to Valachev. Since pVlad plays like a super solo anyway, this just gives you more options for mercenary units to take with him, although Boomhowler & Co. might be great for providing a fantastic tarpit that could also pack a whallop.
eVlad - Feat: Blood Legacy, Hand of Fate, Transference - eVlad loves faction models and mercenaries will certainly get a treat from him. While Transference will not be nearly as effective as it is with Blaize, it's still nice to benefit from boosts with infantry - especially since Hand of Fate can further improve their offensive output. On top of that, they can also benefit from eVlad's feat, if you decide that those models are deserving of the 'Blood Legacy', or would be in a position to make the best use of the stat boost, considering that they could also Zephyr that same turn.
pButcher - Feat: Blood Frenzy - Fairly straight-forward here. Gaining an additional die on attack damage rolls is nothing to scoff at, especially with ranged units. With Iron Flesh, Croe's Cutthroats can be an absolutely brutal unit. They have a high enough DEF that they can survive on the flank long enough to get behind the enemy, and with Zephyr they have an even better chance of getting behind targets for Backstab (although if their activation has already started, they can't get the back-strike bonus). On the feat turn, each member of this unit has the potential to have 6d6 for their damage roll (2d6+d6 [feat]+d6 [charge]+d6 [Poison]+d6 [Backstab])!!!!
eButcher - Feat: Feel the Hate - eButcher's feat is often better used on solos rather than units, but I don't think anyone realizes how much fun you could have with pAlexia and her Risen. With Zephyr she can get into a really great position to threaten a range of targets and then when feat time comes around, you can give her rage tokens that she can spend to advance deep into enemy lines since she could just sacrifice her Risen for boosts and even extra attacks, since it doesn't specify that it needs to be her activation to spend them! This means that Alexia can go on a bloody rampage all by herself, with boosted and extra attacks all along the way, as long as her Risen stay nearby. Yikes!
Of the factions with MUAs, Menoth is probably the faction that needed them the least (probably why they have FA: 2 on their Priests). The Protectorate already has a fairly broad selection of units to choose from that run the spectrum of cheap to expensive anyway, and they all work fairly well with the warcasters without there being a huge need for outside assistance. There are, however, a couple interesting implications for those warcasters who have spells or abilities that target friendly faction units, that now can affect mercenaries.
Harbinger - Martyrdom, Crusader's Call - Harbinger could actually use some Mercenaries to add to her cadre. Her Martyrdom ability would be great with a unit that naturally receives tough like some Press Gangers to create a cheap and sturdy line and you can still bring Rhupert to make a more valuable unit tough as well (Errants come to mind). Much like Blaize, Crusader's Call is great for a Mercenary unit too, and you could launch a unit like Halberdiers quite far upfield to deliver their payload - especially since they can gain Pathfinder from their Priest.
Vindictus - True Path - Considering that the Priest already can grant pathfinder, Vindictus probably doesn't want to run with Mercenaries unless you're facing a spell-heavy opponent.
Thyra - Feat: Shadows of Urcaen, Carnage - Thyra will run faction merc units fairly well, granting them the extra 2" placement during her feat turn and they can benefit from the +2 to their attack rolls. Steelheads on the charge can now be at quite a silly MAT that can threaten even high-DEF targets with ease.
eKreoss - Imperishable Conviction - eKreoss doesn't particularly favor Mercenaries with an attached Priest, but if/when they die, you can heal him up.
Reclaimer - Feat: Resurrection, Reclaim - Having friendly faction models around to fuel the Reclaimer can be quite a boon! This offers you some flexibility if you choose a unit like Halberdiers that pack a little more 'oomph' in melee, because they are cheap and you can bring them back via Resurrection and have them threaten a wider range of targets between their higher P+S attacks and Powerful Charge.
Testament of Menoth - Reclaim - Unlike his previous version, ToM actually doesn't particularly care for Mercs since he'll typically have access to cheap souls anyway, and would rather revive targets with damage boxes. However, you could be really sneaky by taking Croe's Cutthroats and running them along a flank (protected from spells), hoping to score some backstrikes or at least put pressure on the opponent's warcaster/warlock.
eSeverius - Holy Ward - While Holy Ward targets a 'friendly faction' model/unit, it'll probably get more use on other targets besides Mercenaries since the protection from spells is a little redundant with the Attendant Priest already capable of granting it. Unless you're attached to a unit that desperately needs Pathfinder or Magical Weapons, eSeverius will probably want to toss the spell on another target entirely.
Come Monday, I'll have three lists made up that feature mercenary units and their MUAs for you to look at and consider. I hope that this thorough look at each warcaster will at least get you to reconsider using MUAs, as they at least give you options for alternative playstyles, and I think that's the strength of this game over others. If you don't explore your options, then you miss out on one of the greatest aspects of Warmachine/Hordes!