Nasty Plot Mew's coverage isn't THAT strong. SE moves with low base power still have low base power. Victini for example has V-create, which is 270 after STAB. A more common bp is 120 or 180 after STAB. Mew's Psychic is 135, SE Aura Sphere is 180 and SE Shadow Ball is 160. So what about the stuff that Ghost/Fighting doesn't hit super effectively? You have to rely on that pitiful STAB.
252 +2 SpAtk Life Orb Mew Psyshock vs 0 HP/0 Def Shaymin: 83.58% - 97.95% (Psychic=59% chance to KO)
252 +2 SpAtk Life Orb Mew Psychic vs 252 HP/0 SpDef Milotic: 67.01% - 79.19%
You aren't getting KOs on neutral targets and that unboosted speed means you're easily revenge killed. I'm aware mew is bulky enough to take a hit, so you have to let something die to kill it. Anything with Specs or a Band has the same effect, without the turn of set up. Mew should not be banned.