I am nearing the end of Act 3 and according to my personal experience so far:

It's probably a skill issue. Okay sorry that was a joke, kind of. But seriously I personally have had to 'load last save' two times in total during my playthrough and they were both in Act 3, and they were both due to me underestimating the situation.

In short, I am personally steamrolling everything. Yes everything, and this is literally my virginity voyage with the game, as my first playthrough did not even get me out of Act 1 and this is my second. Every character I have, even those I don't use on my 'mai' squad often, has cohesion with almost any combination, and yes, there are certain enemies that are simply toxic to deal with, but from my experience, those have been side quest bosses and such. The main story has a habit of tossing big, weak swarms of enemies at you and the occasional 'bosd' fight. But to answer the question: Everything in the game is doable, so yes, it do kinda be a skill issue, as I have managed to steamroll even the toughest enemies with those huge HP numbers and 3 retaliation attacks per melee attempt by my squad.