It's weird. I have had exactly one berzerker my whole game. They were introduced months ago in-game. I do like how there's so many enemies now that you can go awhile before seeing them again. I remember Mutons showed up for a few missions then disappeared entirely, only to now start showing up again after months of absence.
But yeah. I think a week or two after I upgraded from Kevlar I started getting Archons, and figured worse was shortly around the corner so I started playing catch up on weapons and armor. I don't find Gate Keepers too bad with Mission Timers +4. It's enough time to get in position and get a good round of concentrated fire on them. Bluescreen Rounds and EMP grenades make short work of them. I never leave home without them anymore, now that Sectopods are an every mission occurrence. ><
TBH other than Alien Hunters, and excusing modded turn timers, I don't know if the game is especially hard. Except for those Defend missions, jesus. With all the enemies rolled out you get like....2 turns to engage everything attacking the transmitter, and even when engaged half the enemies will look at you, then turn and destroy it rather than fighting.
But eventually I got frustrated as every level was costing me half my men, or I was losing, and the AVATAR project was saying "No you cannot be a scrappy underdog forever".
It's the wound times that were getting me, personally. Having half your squad at at a time going 15 to 30 days injured was just becoming too much. AWS and what not helped but really the increased HP from armor is started tipping the balance back.
I also like the game more w/o the Avatar project. I approve of the Dark Events, but I hate the "you can't have fun anymore" timer. I've got the Avatar Project modded to have like 2.5x more steps to it, so it's basically a non-issue but I may just disable it entirely in a new playthrough.