Mega Gardevoir is a pretty solid Pokemon in its own right. Its biggest flaw is its low physical bulk, but other than that, it has a lot going for it. Its special bulk is pretty spectacular, a little better than an equally invested Mew. This makes it a pretty nice check to stuff like Life Orb Latios, provided you toss a few EVs into Def to prevent Psyshock from 2HKOing. Fairy typing is just fantastic since it removes its former Bug and Dark weaknesses, makes it even more resistant to Fighting attacks, and gives it that infamous Dragon immunity. It's pretty great on the offense, too. Pixilate Hyper Voices coming off a base 165 SpA puts huge holes into pretty much anything that doesn't resist it or isn't ridiculously specially bulky; even specially defensive Rotom-W is cleanly 2HKOed with no prior damage whatsoever. Its support movepool gives it a lot of options as well, whether it be Will-O-Wisp, Taunt, Destiny Bond, and Healing Wish.
The main problem with Mega Gardevoir isn't so much its own flaws, more the opportunity cost that comes with using it. When you're using Mega Gardevoir, you're usually going to want to use it as a wallbreaker since defensive sets are generally overshadowed by Sylveon and Clefable. However, there are already several other Mega forms in OU that are capable of punching holes in teams, such as Mega Charizard Y and Mega Pinsir, which give Mega Gardevoir a lot of competition. When you use Mega Gardevoir, you're giving up the chance to use one of these other powerful Mega forms, so that's where the opportunity cost comes in. However, it's perfectly capable of pulling its weight plus some. Heck, it was even banned from UU Beta because it's just that powerful.
