Yes, Mega Gengar does constrain teams. It's like how Blaziken and Excadrill in BW1 constrained teams. Blaze and Drill forced people to run very unorthodox physical walls (Bronzong fell to UU AFTER Exca's ban). Similarly, Mega Gengar constrains teams. It shits on a lot of teams with no effort, and even if 50% of your team laughs at it, it has Shadow Tag to eliminate the other 50% quickly. It's not just that your team can't be weak to Mega Gengar, but no more than 1 or 2 of your mons must be weak to it, which is a giant demand.Maybe I just build the right teams, or something, because I've never had trouble with MegaGengar. Hell, a pokemon like Ninjask can deal with him thanks to night slash infiltrator outspeed (but I guess that's a moot point since 99.8% of OU players wouldn't consider using a pokemon like that anymore when they could just fill up their team with Ferrothorns and Ageislashes).
If MGengar is getting too strong, maybe it's time the metagame shift? I think a quickban is stupid, when the metagame is still growing up. If MGengar is too good, let's find out after three months of play. If people can't figure out how to deal with something as simple as a fragile ghost pokemon by then, go right ahead.
People put up with Stealth Rocks being allowed in all tiers, which severally constrain the number of usable viable pokemon by at least 75%. MGengar doesn't hardly constrain teams at all, in comparison to that.
Stealth Rock hurt some Pokemon (Victini, Arcanine, Moltres, Scyther, etc.). But it didn't have nearly the impact you described (Volcarona and Dragonite will still be Volcarona and Dragonite, Ho-Oh will be Ho-Oh, etc.).
Also, yes. Most people cannot deal with Mega Gengar, because it can switch and YOU CAN'T. In order to have a team that completely beats it you have to be against a really stupid opponent or have something like Tyranitar/Scizor/(you get the point).
Mega Gengar can do so much other than Perish Song. Shadow Ball+Sludge Bomb/Wave+Focus Blast+DBond/Taunt/Sub is a really deadly and really common set, and there's a crapton of other moves it can run to screw over mons expecting those moves (I've seen Dazzling Gleam pre-Pokebank for Chesnaught, and, well, almost everything). Also if you're against a Vest user that can hurt you badly, switch.Imo perish song + shadow tag is the thing that should be quick banned, not into ubers but from the game in general. After that then we can do a suspect test or observe how MegaGar fares as a special sweeper. Imo MegaGar is helpless against Vest users, if not for its destiny bond.