My biggest pet peeve about the Prometheans, definitely. They are just cheap. There's one part on Infinity where you need to hold off a bunch of them while Cortana opens a door, with no cover, no ammo, and you get Crawlers in multiple waves of thirty or so, as well as Knights who are apparently not solid as Railgun rounds pass straight through them. It's stupid on Normal, I can't imagine what it'd be like on Legendary.
And these hold true pretty much every time you face fucking Crawlers. It's never 5 or 6, it's always in at least the high teens. What they lack in actual intelligence they make up for in sheer numbers and firepower and speed. They're like kamikaze Skirmishers crossed with Combat Forms.