If Mega Metagross doesn't run HP Fire, Mega Scizor can come in on any of its attacks, set up in its face, and sweep while Roosting off the damage. I won't be losing any offensive presense if I know a would-be major threat like Mega Metagross will give me free set up, at which point +6 Attack uninvested will still be good enough to sweep most teams.
0 SpA Mega Metagross Hidden Power Fire vs. 248 HP / 228+ SpD Mega Scizor: 136-160 (39.6 - 46.6%) -- 21.1% chance to 2HKO after Stealth Rock
(78.9% chance to 3HKO)
Don't even try to argue the fact that MegaGross is nothing but set-up bait for one of the most common Pokemon in OU, which is one of the criteria for something to be unbanworthy. Saying that a Pokemon has checks and counters doesn't stop them from being non-broken, but a Pokemon is broken if, in a given metagame, it is a dominating force and dictates playstyles to such an extent as to be majorly centralizing. MegaGross has relevent counters that are all in the same tier as it and it has numerous relevent checks that are also in the same tier. That means playstyles barely change from its presence. Standard Mew has a field day with it. That is the point of my argument, and the reason I stand firmly on the belief that Mega Metagross should not be banned.
I did not even argue that Mega Metagross is not set up on by Mega Scizor but showing that it does doesn't prove anything, really. Sure, Mega Scizor is good and all but what if I want to use some other Mega, say Mega Altaria?
Standard Mew can take its hit, fine but it can't really do anything back except Will-O-Wisp which a teammate will come in and force Mew out. Then, if Mega Metagross comes in again, Mew will not be able to take its hits anymore.
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 252 HP / 224+ Def Mew: 157-186 (38.8 - 46%) -- guaranteed 3HKO after Stealth Rock and Leftovers recovery.
Mega Sableye is a really shaky check since it can't switch in so you need to sac a Pokemon for it. Then, you are forced to Will-o-Wisp (otherwise you risk being 2HKOed) which the opponent can then use to switch and force Mega Eye out.
As for Talonflame, it is easily worn down by Stealth Rock and the fact that it's moves cause huge recoil on itself. Furthermore, it can't switch in since Zen Headbutt cripples it to the point that it needs to Roost.
252 Atk Tough Claws Mega Metagross Zen Headbutt vs. 80 HP / 0 Def Talonflame: 249-294 (78.5 - 92.7%) -- guaranteed OHKO after Stealth Rock
Finally, Landorus-T and Garchomp. Lando-T relies on Intimidate to check physical threats and that can be mitigated with Clear Body so all of Metagross's moves will sting. And both are outsped (unless Scarfed) and 2HKOed by Meteor Mash and flat out OHKO-ed by Ice Punch.
252 Atk Metagross Meteor Mash vs. 252 HP / 240+ Def Landorus-T: 118-141 (30.8 - 36.9%)
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 252 HP / 240+ Def Landorus-T: 166-196 (43.4 - 51.3%) -- 62.9% chance to 2HKO after Stealth Rock and Leftovers recovery
Nobody is arguing Mega Metagross is invincible and all (since this is 6 vs 6 not Mega Metagross vs The World) but the fact that it can completely steamroll teams lacking its checks while having the potential to screw over its checks with the right move is what makes it broken.