From what I've noticed in the many games I've played lately, Chandelure and Heracross are much more threatening than Mew as stand-alone Pokes. Chandelure 2HKOs almost everything in the tier with its STABs, and cripples the thing it can't kill with Trick. Heracross easily sweeps teams with one of its STABs after you kill the other team's Gligar or Cofagrigus; I've won way too many games by just spamming Scarf Megahorn.
Mew can be threatening against certain teams, but almost always needs a turn of set-up to do so, since it doesn't have the raw stats to hit things hard off the bat. That's easier said than done, especially in a fast-paced meta with powerful things like Darmanitan, Victini and Porygon-Z.
I've rarely seen Mew do anything broken, so it might just be a paper vs. practice thing. In any case, there are other things that we should look at before we ban Mew.