Mistress had a combination of bad design decisions, but some ranged DPS didn't have to deal with them, as much. The camera limitation designs this xpac were noticeable on many fights, but nowhere near as much as her. This was annoying for anyone that plays zoomed out farther than short to mid-range. Add to that, the translucent floor, and 100% miss chance void zone that you can hardly see, especially where it ends, and it was the dumbest thing to deal with for tanks and melee. Then, it was tuned tight enough, mythic was taking 150+ wipes; making a potent recipe for shit tier boss.
I guess Kingeroth was a bit of the opposite? Good for a few classes (that have short dps cooldowns), and shit tier for everyone else? But hey, at least the camera wasnt doing stupid shit, so its got that going for it.