Mega Metagross is already banned to the Ubers tier.
Charizard-X is a nice DD sweeper, but has definitely its flaws: Before mega-evolving, it is 4x weak to Stealth Rock, so that it loses 50% of its health on the switch-in, and after mega-evolving it is still 2x weak to rocks. The recoil of Flare Blitz quickly wears it down, and if it chooses to run Roost over Earthquake, it is walled by Heatran, Tapu Fini and others. Also, it is actually very easy to revenge with Scarfchomp, if it only gets one boost.