What really annoys me is when romhacks deck all the trainers' pokemon out with things like focus sashes, choice scarfs, choice bands, etc. You know, the items that you can't even acquire until post-game at the battle tower/frontier/whatever.
I don't have a problem with trainers using these items, but at least make it so I can access them to, if you're going to do that.
Also ridiculous buffs to random-ass pokemon. Drayano's hacks are great, but I hate shit like turning Farfetch'd into an OU-tier sweeper inexplicably with flying/fighting typing, 120 base stats in attack/speed, and then having to fight against it in the level 10-20 range with your unevolved pokemon. It wouldn't be so bad if you knew it was coming. Instead I get completely curbstomped by one, mutter "dude what the fuck" to myself, and then go check the changes.txt file only to then find out how ridiculously buffed the bird is.