Eh, I could see why people don't like them:
Not Mega Evolution: For those who like Mega Evolution, even though they're still in the game, they probably see Z-Moves having taken their prominence.
One Use: Sort or relating back above, the fact it's a one-use power-up probably gave a lot of players feeling a lack of punch, especially if it didn't knock out the opponent (at least Mega Evos lasted until the Pokemon fainted). And if protected against/nullified you wasted your big move and now have no item to boot.
Long Cutscene: Sadly a problem that still exists to this day, we get it, this is a flashy power-up, we don't need to see the flashy transformation sequence. This isn't Power Rangers of Sailor Moon where you got to fill the runtime, we can skip/shorten the scene.
I think GF is maybe approaching an "ideal" when it comes to super mechanics, something that gives all Pokemon a temporary power boost, and some even a super form. However Dynamax and Gigantamax still aren't the answer I feel, they're too gimmicky with the whole "kaiju" concept plus are still only once per turn. I think something like a "power bar" could work, something that fills up for the player during the battle and when filled at certain points the player can choose to have one of their Pokemon use it to either go into a super form for a few turns, use a super attack (maybe having said move be different power/effects depending on how much the bar you have filled), & maybe some other things, and then you start to fill it again. Also I say not for them to be afraid to adjust how certain super forms and super moves work to fit into a new system, like instead of increasing base stats just have Mega Evolutions give certain stat boosts & shrink down the Gigantamax forms.
