>>24885203They're the embodiment of properly allocated stats but in hideous forms. It's mainly to do with Prankster, Sheer Force, and their Therian formes.
Lando-I with Sand Rush is fine. Thundurus-I and Tornadus-I with Defiant are fine. Prankster is generally balanced on Torn, but on Thund, Thunder Wave is too cheap. It's the same with Tail Glow Manaphy. Something like Volbeat that gets both Prankster T-Wave and Tail Glow is much more balanced, but when you give these features to high stat Pokemon with much more in their arsenal, it's just dumb because of how centralizing it makes things.
Regarding the Therian formes, Gen V had a lot of problems welcoming the power creep. You had 140+ offense mons like Darmanitan, Chandelure, and Haxorus so they thought it would be ok to do the same to the genies, putting their offense (and speed in Torn-T's case) toward the height of their stat tiers. Their movepool was one thing, but incredible stat distributions made them a pain to play around.
I'd go on but I have to leave, but this is part of the reason people don't like them, basically. Unfair, ugly, and too useful.