Besides the intro level where I was almost at max level of hive infestation I think my most hated level was the Nostromo knock off. My gamma setting was all f**ked up, so it was dark even when the lights were on, there were aliens EVERYWHERE that I kept setting off, I finally left with a beat up squad and came back to a glitch where the lights were off again and it made it hard to navigate.
I think my whole journey would've been easier had I known that the infestation level was making the game harder. I knew there had to be some kind of consequence, I just didn't know what it was (me read the tutorial lololol). I thought maybe it would make a final hive worse to fight or something. I ended up making the first queen fight so tough because it was queen battle, onslaught, type 2 xenomorph, with more than typical aliens respawning in between.
I literally had one Marine limp back to the elevator to beat it.