While I like Mega Evolution, it has a few shortcomings that prevent it from being a balanced mechanic with interesting lore. It's exclusive to only a select few Pokémon (about 46), with 2 of them receiving 2 Megas each.

Almost none of the new Pokémon that Megas were introduced alongside could take advantage of the mechanic, and the only one that could was from a limited time event. It clearly favored the old rather than the new, especially since a lot of the Megas in XY were from Kanto.

Most of the Pokémon that received one were either already powerful enough and didn't need one, like some psuedolegends and RAYQUAZA. And the most of the weak ones that DID get one were either still not good enough or became incredibly overpowered like Kangaskhan and Mawile.

It wasn't used enough in the game it was introduced in, where 4 major npc trainers had that power. Getting each Mega Evolution was also a bit annoying because it requires 1 species that could Mega Evolve as well as their own species-specific stone to use it.

As a battle mechanic, the fact that it boosts stat totals by +100 for as long as the Pokémon is alive is overpowered. This led to a power creep where almost everyone had a Mega on their team. It got chaotic in Gen 7 when Z-Moves and Megas were in the same game ON THE SAME TEAM.

Mega Evolution, as a concept, is really cool! I just wish they utilized it better in XY and used it more. And what sucks the most is that they left behind some cool designs that would never be brought back because it's attached to an unbalanced battle mechanic!