Well, the choice to make AI hordes braindead zombies who hust mindlessly storm you, is confusing to me as a player, too, if the enemie are supposed to be not zombies, but smart special forces.
Titanfall is a pretty good example of how to design good AI. Same goes for Age of Empires, I would say. XCOM, on the other hand, just increases the HP and count of enemies, which is a bad experience.
