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Mewtwo unfortunately suffers from a mediocre typing within the tier's context (mostly base and MMY). MMY also faces competition from other offensive Mega Evolutions, and that argument applies even more so to MMX, who has to compete with Mega Salamence as a physically offensive Mega. Their...
Banning is a whole other procedure in and of itself which is decided on by the tier's higher-ups. Plus, why not give the meta some time to settle first? Observations are never a bad thing, but don't be hasty with ban calls, please.
Hope I'm not being dumb, but you've stated in the recent OU VR update that Zygarde and Ash-Gren have risen to S. Their position hasn't changed yet, though, so I'm a little confused. Just thought I'd inquire.
Arcanine is still UU while Entei has dropped to RU, and Jolteon is still comfortably RU.
Other than that, the new NU and RU drops are looking pretty solid.