Honestly, I can see it getting banned. With coverage in the form of Zen Headbutt, EQ, the rare Pursuit and Hammer Arm, there really aren't a lot of things that can actively switch into it and force it out. If they predict a switch and use Agility, it's almost GG if you don't have something like Gliscor or a Garchomp with Rough Skin at 76% or higher HP when it comes in. It does restrict team building a lot too, as one of the Pokemon that could be a better check to it (Mega Swampert) isn't because of Grass Knot. Seen a couple sets utilize Sticky Web so that they can invest in Bulk, and with a Hone Claws boost it can rampage through teams that don't have something to specifically check it as +1 Hone Claws Meteor Mash is brutal.
Now it's W-o-W bait which when burned makes it a lot less of a problem, but not to many Pokemon want to chance a switch in to burn it, and it almost certainly will faint trying to burn M-Metagross. The only thing that really can put a wrench in M-Metagross is M-Sableye and Air Balloon Heatran (if it lacks Hammer Arm), but that forces you to be in the right place at the right time, and is situational. Same with Lando and Lando-T, if it doesn't have Ice Punch and a Agility up, they have a chance (specifically Lando-T if it can intimidate after it Mega Evolves), but that is still a toss up.
I like Mega Metagross a lot, but if it gets banned I wouldn't be surprised. It just can hit so many things amazingly hard, and just doesn't seem to have enough viable checks to it.