>>55956829As designs, mega-evolutions were never good. The vast majority of them are lazy and add nothing to the mons granted.
As tools, mega-evolutions were questionable hedges against progressive generational powercreep, but even then they're typically lazy buffs that don't open up new space in terms of how to use the species.
Take Glalie for example. You lose Moody, which at its most optimal plays completely differently from any other Glalie ability, just to gain Refrigerate which is the same normal move spam as its companion abilities with the Aerilate and Pixellate mons.
There are only a few exceptions, like Mega-Banette maybe, Mega-Sableye especially, Mega-Gengar maybe, and Mega-Lopunny only because the original was so weak it couldn't do what the mega does.
Megas could have been so much more interesting than they are, but the execution was botched.