@@Outkin,
No, not really. It is no different than using a Legendary, or Pseudo-Legendary. They all have their strengths and weaknesses. And it all depends on how you use them. As you can only have 1 mega-evolved pokemon in any battle at any time, that is 1 way they balance it. And if the pokemon faints, but was revived; they can't re-mega evolve.
Some of the megas are better at certain roles than others, but they all have a place, and they all have presence. But they are not super Op. As long as you play smart, you can beat them.Though I'd say Mega charizard y is quite a powerful mega as he gets drought. So that makes sunny day teams super easy to manage. and then there are some obvious threatening megas like Garchomp, Scizor, and Gengar for example.
Also, just because I feel like mentioning my current team for competitive play; I like to use the following:
Goodra: mixed attacker
Charizard Y: Special attack sweep
Sylveon; Wish, Baton passing special defence wall
Avallug: Physical Tank, Rapid spinner
Scizor: Physical sweeper
Tyrantrum: Physical sweeper, stealth rock
I find this team pulls through quite a few team fights with the broad type spread, and useful stabs each can provide. I plan to get a new Physical wall because Avallug has very VERY low special defence, and can't take any kind of hit from such attacks. And with him being just ice, that does give him a few weaknesses that can be easily exploited.