Vehicle Fights:
I hate all vehicle fights. I want to play my class, not a 4 button vehicle.
There should be a special place in hell for whoever came up with Ambershaper, because the learning curve required wiping 1-5 times per person, with RNG on who got to drive. That fight would have been 10x easier if you could just pick who got to drive and they were permanently the vehicle. Even if they just put a practice vehicle next to the boss so everyone could fix up their UI and learn how the buttons worked it would have saved incredible amounts of frustration.
Gimmick Fights:
I generally like fights with non-standard mechanics as the driving element of difficulty. Tossing the ball on H-Sha, throwing turtles on Tortos, nests on Ji-kun just to name a few from this expansion. The real frustration of these mechanics is when they're too RNG or the learning curve is personal rather than raid-wide and it takes FOREVER for every single person to learn the mechanics.