Pokémon X and Pokémon Y

I can't exactly see these pokemon MegaEvolving in every single battle, there must be a limiting factor to it in my opinion.

The limiting factor is they can't use other hold items. That's a really big crutch so it depends on how strong they are compared to regular pokemon.

Serebii also says you can only have one mega pokemon on a team at a time.

Maybe it'll revert back when you switch it out?

Serebii says it only happens at the end of a battle, explicitly stating it's not when you switch it out.