Mew is banned because you can't counter it until you know the moveset, by which time it probably has already set up, and because even its counters find it goddam hard to kill.Why is this even being considered? Zapdos, Snorlax, Swampert, Milotic and Salamence are all whored way more then both Celebi and Jirachi. Every frigging bloody team has them and a poke that you might throw in for kicks gets banned? Kinda silly to me. In fact, I think it's silly that Mew is banned. Mew was only uber in R/B/Y and it's hardly a threat in advance. Then again, does it really matter? D/P is already out and these pokes are even weaker there when you compare them to the "soon-to-become-the-new-pokes-whored-more-then-the-average-hungarian-street-girl".
Gengar isn't nearly as durable as Mew, and it can't really set up anything besides a Sub. Tyranitar sports a few common weaknesses to Water, Ground, and Fighting.Doesn't that goes the same for anything...you can't counter Gengar/Tyranitar until you find out it's sets....
All Mew's going to use is either SD or CM set, and SD set, with no stab moves, won't do shit to solid tanks such as Skarmory, since one of the perks of using Mew is probably SD Explosion. And if it's a CM'er, it as always is still walled by Snorlax and co.