What support does it need, help on the Physical side? So do so many things, just because it has like two huge counters, so does the top threat, Genesect (Heatran & Gastrodon), besides, Heatran makes the perfect partner for it, and it isn't just a pushover because of the Defense, people in general have to stop focusing on one low point versus a lot more.
Actually, MegaVoir is a pretty big pushover to any physical attacker that can strike before it, and that would include priority moves. Earthquake, Stone Edge, Brave Bird, U-turn, Crunch, Return, Foul Play, Shadow Sneak, Bullet Punch, Aqua Jet....pretty much any unresisted physical attack you'd care to name will dent MegaVoir, especially considering the lack of recovery. Like I said, this wouldn't such a huge deal if MegaVoir can outrun a huge majority of opponents, but 100 Speed with no way to bypass it is actually really damn slow (Volcarona can Quiver Dance, Mega Medicham can at least Bullet Punch something...).
Besides, Gardevoir has a large support movepool, like Will-O-Wisp (which I'm considering running), Wish (albeit only 139 HP though), Dual Screens, Healing Wish, Memento, Thunder Wave, hell, it even gets Sunny Day and Rain Dance if you want that!
It's a nice support movepool that usually no one wants to waste their Mega on (hence the disappointing lack of Mega Banette). Regular Gardevoir can do these just fine.
And the fact it dies to any Bullet Punch (252 Atk Adaptability Mega Lucario Bullet Punch vs. 4 HP / 0 Def Mega Gardevoir: 272-320 (97.8 - 115.1%) -- 81.3% chance to OHKO), so do every other thing weak to Steel, of course Mega Gardevoir just like everything else is going to fear a Super Effective move, and how common is running Steel-type moves anyways?
It's not just Steel moves Garde has to worry about: every unresisted physical attack of decent power can easily 2HKO MegaVoir. This is a powerful special attacker with only okay speed but has Blissey-like vulnerability.
I mean Mega Charizard Y has a shit typing, but it is very powerful, and Gardevoir has a very nice typing, it only has three uncommon weaknesses (Ghost is only prominent in two Pokemon in OU really, it was so great, but that hype has settled, it was just Aegislash), with an immunity to Dragon-type, and a 4x resistance to Fighting is huge honestly, as those types are still great. Overall, Mega Gardevoir is certainly not C Tier lol, it is definitely B+ and above.
YZard (and Talonflame)'s typing is actually not that bad: only Stealth Rock is the major pain, but it resists a ton of shit that not too many Pokemon would carry coverage moves for (bulky Grounds like Hippowdon and Gliscor nowadays don't carry Rock moves, non-Azumarill Fairies don't normally prepare for them, Fighting-types favor other coverage like Dark or perhaps Ice....), not to mention YZard can easily remove one of its weaknesses with Drought. MegaVoir does have a nice Fighting resistance, but it would honestly be better off without Psychic-typing as it would like the Dark and Bug resists and shed its Ghost weakness (the Psychic moves themselves still complement Fairy nicely).