I see a few people arguing match ups and what Gengar can and cannot kill. Let's establish a few things first:
1. Before mega evolving, Gengar is a completely viable threat. It can send off a solid attack on the turn it mega evolves. In no way is it a liability on this turn, because people had found ample opportunity to use Gengar in previous gens.
2. After it has mega evolved, Gengar becomes a fantastic support pokemon. When I make a team with mega Gengar, the most important question is "what pokemon can Gengar kill?" I choose this, not the opponent. Whether this be through Perish Song or 4 attacks is up to the team. Once I've established that, the next question is "Once those pokemon are eliminated, is there any pokemon that can sweep, after 1 turn of a set-up, the entire metagame?" Up till now, everything I said should be applicable to most people. After this is completely my opinion.
I do not believe there is a pokemon that can use the holes provided by Gengar to sweep unconditionally (or even under most conditions). If this was last gen, I would believe Gengar is broken immediately, but this gen has some complications, namely the introduction of mega evolutions. In the hands of a competent player, Gengar can take down 2 pokemon, most of the time. To me this WOULD have been broken, but it is so no longer. Gamefreak, rather idiotically, decided to introduce a subset of pokemon that are superior to most other pokemon (the megas, or more accurately, a decent amount of megas). With a good number of them, you can take down at least two opposing pokemon, or sweep weakened pokemon, far easier than you could with most pokemon. The "down side" is you can only use one. This one IS more important than the member of your team. While teams don't necessarily have to be built around them, they are more pivotal than regular. pokemon. For this reason, I don't believe mega Gengar is broken; it is simply the most obvious example of this new kind of pokemon. I am conflicted though, and wouldn't be upset either way.