One big difference, like I mentioned, is deployment. There are actually only 4 scenarios that utilize the Reinforcements artifice and they are:
As you can see, the deployment for these scenarios is a little different (except for Outflank), where your normal deployment zone is squished into a corner - almost like radial deployment. Be careful, however, as you'll also notice that the reinforcement line is where the reinforcements enter from, and they get placed within (note: not completely within!) 3" of this line. This means that large-based models like heavy warjacks/warbeasts will make the most of positioning as reinforcements and will give them a better threat range, so it's important to pay attention to how and where your other models. Also remember that reinforcements can enter at the start of any turn, starting on turn two. Many people bring in their reinforcements right away, but this might not be the best time to do this, depending on what else is around.
The next thing to consider is that two of the three flanking scenarios have the scoring zones closest to your reinforcement line. This means that your main force will be expected to directly face whatever reinforcements your opponent might have. In other words, your reinforcements should be capable of standing on it's own against your opponent's main force, or at least be good at scoring in the zone or dislodging models from the zone to give the rest of your force an edge. Consequently, your reinforcements should have some defensive capabilities or be difficult to remove to make the most of its presence.
When choosing reinforcements you should also remember that character restriction still applies! That means that taking a character as a reinforcement means that it cannot appear in another list - reinforcement or otherwise! Lastly, remember that time is vital in these scenarios, and while you get an extra turn extension for reinforcement scenarios, you don't get any extra time in Death Clock, so having a large unit with lots of attacks can actually be a liability in flanking scenarios. In other words, you might not want that WGDS as a reinforcement.
With these considerations in mind, let's look at two options for consideration in the Circle Orboros, since it's my main faction:
Option A) Druids of Orboros - A unit that fills the 7 point slot with ease. These guys offer some great utility for reinforcements since they can use Force Bolt to move enemy models out of scoring areas and/or closer to your models to wreck. Additionally, Magic Vortex turns them into a great tarpit as well, making them DEF 16 against non-reach melee attacks, and DEF 18 against ranged/magic attacks. However, with all of their attacks and laying down cloud templates, they are less than ideal because they can chew up a lot of time on your clock compared to other choices like...
|COME AT ME BRO|
Option B) Shadowhorn Satyr - This warbeast is oft maligned in favor of other, more effective warbeasts (like Warpwolves) but I think finds a special home as a reinforcement. One of his main weaknesses as a warbeast is that he unfortunately is pretty pillow-fisted, often only hitting as hard as a light warbeast, but what he offers is utility - not brute strength. With the ability to leap over models, his threat range when he arrives is pretty huge (especially since he only has to place within 3" of the edge). While he won't beat-down many models, he can two-hand throw those models towards the rest of your army to do the damage. Lastly, he's also pretty annoying to deal with, with set defense and reversal, making your opponent think twice about charging him, or devoting more resources to deal with him that would've been put towards the rest of your force otherwise. Lastly, as he's only one model, he won't chew up much extra time on the clock and is totally worth taking.