I'm wondering if PP will fall into the XCOM pitfall
I'm not sure it's a pitfall? If the enemies didn't get tougher, then the game would just get easier and easier as you progressed, which means if you beat the first mission few missions then you can basically walk the rest of the game.
I suppose difficulty could be increased in other ways such as more enemies instead of tougher ones, but if you can one shot anything and a single grenade could wipe out a large batch of enemies then the game would be too easy.
I've found this on most of my XCOM playthroughs, if you get through the first few missions the rest is plain sailing, if anything I don't think XCOM cranks up the difficulty enough.