Isn't it a bit soon to be banning anything?
I mean, it sounds like they have to give up their held item for mega evo, which is a big deal in competitive battling. Pokemon without Choice Specs, Focus Sash, Leftovers etc.
Not if the Mega Pokémon in question can decimate a full team of six with the boosted stats.
Unless GF hasn't said that Mega Evolutions do decrease stats to ridiculously low numbers...