Mega-Gengar has a problem in that what he can get rid of is limited to the set which he is running. The Perish Song trap is flawed in that anything that can take it to under 25% HP either by itself or with the help of SR or previous damage means that it can't run the last Sub, but still do a nice job of getting rid of certain Pokemon. Sets like
MikeDecIsHere's compliment the team very well against defensive threats for other sweepers, but it can then lose its ability to defeat some of the strongest Specially Defensive walls (Blissy, Chansey etc.), though I guess at that stage it is beyond the point. Both these sets though are very strong team supports and really good at shutting down
walls. M-Geng isn't as able though to take down
sweepers unless it is running Destiny Bond and moves first. Getting rid of your opponents walls do not always call for an instant win, and the late game battle of the sweepers and setups can still beat team. Sweepers can sweep other sweepers, and M-Geng can at times give them chance to sweep if instead of going to revenge kill M-Geng I predict it to switch out and setup up a Dragon Dance or whatever on my Dragonite, or a SD on my Scizor for example, allowing me to wreck significant portions of your team. Its all a about good prediction and a lot of other things. Granted Mega-Gengar pokes giant holes in defensive and stall teams but can also do rather poorly against other offensive teams which M-Gengar fails to take out. And likewise although other Pokemon cannot trap something like Mega-Gengar they can threaten huge damage to whatever switches in which does a very good job, only a little inferior to Mega-Geng. I know that this is a little situational in a couple of ways, but once you know M-Gengs set you can play around it.
But stall teams don't have to worry too much, as M-Gengar isn't on every team, which is due to people using other Megas. Mega-Charizards, Mega-Kang and Mega-Lucario are all still very popular and M-Geng will still have to compete with them in order to get a spot. This adds a cost to using Mega-Gengar which I think should be played around with more in order to see what it is worth. Mega-Kang can cause problems for a Mega-Geng team and vice versa. It's about each team only has one as a driving force in a role and most teams will be threatened by them. I want to see more into the drawbacks of taking the mega-slot before we go banning them. When we ban one there is more reason to ban another due to the lack of competition. I feel that this new part introduced to Gen VI should be tested more, because I think it is worth more than what people give it credit.