I’ve talked about the Lost, and the Chosen, two pieces that have a tactical change to the way the game plays and make maps a bit more engaging (and stressful). Now I’ll talk about the Rebel factions.
In matching with the Chosen, there are 3 factions. Reapers, who are more like super stealthy snipers. Templars, who are quite good at psionics. Skirmishers, who have good movement skills and can pull enemies out of cover. (sidebar here – Skirmishers work absolute miracles on fast/covered enemies) They apparently hate each other, but I can’t see any game-impacting effects of this. They appear to simply be 3 factions that you can need to gain favor with.
Now it gets a bit more complicated.
You can recruit members of these factions, and rather than have a left/right skill tree, they have more of a talent grid that unlocks over time. You get skill points (AP) per mission, those are used to unlock the items in the tree, once you gain appropriate rank. You can eventually get all those skills… making for uber soldiers. You get 1 of each soldier per faction, and late game you can recruit a 2nd per faction (with appropriate favor).

Reaper skill tree
Let’s talk Reaper for a second, since that’s the first one you get to use. Super sniper, can stay in stealth after attacking (% chance at least), similar skill set as the regular sniper. Heck, they can go back into stealth after attacking with another skill. They can even unload an entire clip into 1 target. More like shadowy death to me…
Each faction has favor (reputation) to gain. More favor, more benefits. You can run Covert Ops for a given faction, using your soldiers for “passive” missions. These missions allow you to increase the rank of soldiers, gain intel, resources, or additional tactical missions (like recovering a taken soldier). Ideally, these should always be running.
Each faction also unlocks a scanning target for Intel, so long gone are the days where there’s nowhere to scan on the planet.
Each month, you have what amounts to a deck of cards to deal to the various factions for additional benefits (Resistance Orders). Could be cheaper soldiers, damage boosts, additional resource gathering, reducing progress on the Avatar project…but you’re limited to the amount of bonuses (cards) based on the total favor with the factions. This is quite useful, and adds a good layer of strategy to the game.
Clearly, there are nothing but benefits to gaining favor with Rebels. But wait, there’s more!
In the base XCOM2 game, there were terror missions where you needed to protect 12-20 civilians from Advent forces. I had a heavy dislike for these missions as the civilians had next to no HP, poor AI, and it was like shooting fish in a barrel on the other side of the map. An early map has a similar mission (the Faceless unlock) but it also comes with Rebels under attack. If you can save them, they in turn help you by attacking the Advent. Their aim is terrible, and the damage is small, but they essentially become target practice for the enemy. This gives a lot more room for your soldiers to do their work. From a mission type I tried to avoid, to one I thought was a ton of fun… that’s an achievement.
Overall, Rebels add both a strategic and tactical change to the game, in nearly every aspect.