I was gonna say 'Megas' but this is closer to what I mean. Megas are still bad though. Most of them should've just been third or second evos.
I'm grouping in Megas, Alolan Forms, and Pokemon Lets Go all together for this one. And the fact that both X/Y and Sun/Moon have more pokemon from gen 1 than any other gen.
Though if we just look at megas alone, I really hated the implementation in game.
First of all, they are mostly given to gen 1 pokemon for X/Y. There are none for the new pokemon that debuted in that generation, or really much of anything for gen 4 and 5, either.
Second, they felt like an instant win button at times (the reason why I like Z-moves better), and makes an already easy game even more piss easy.
And third, nobody else uses them in the rest of the game to even the playing field. NO gym leaders use megas. NO elite 4 members use megas. The only battles in the main campaign that used megas against you were the final rival battle, the final evil boss, and the champion. That's it.
Z-moves were much better implemented. Even random ace-trainers out in the wild could use one. And it feels fair as you can only use it once per battle, so you can't sweep with it like megas.