after reading several post, I think Mgengar is indeed a worth ban, being a stall player, shadow tag with 170SpA and Ghost STAB by itself is already a huge concern the moment I see it. And with the high offensive stats, choice items/LO/leftover is not as necessary as it thinks. And afterall I was not even realizing the vast utility moves it possess.
On the other hand, another thing that I guess many people have overlooked is its immunity to toxic, which makes it even harder to deal with as a stall team, certainly its now extremely fragile against EQ, but for stall teams it don't really make the differences. Taking damage from spikes is nice, but nowhere enough to justify shadow tag. Trading 1 to 1 for a stall team is already a huge loss in most scenario, taking away more than that is disastrous
And for those who argue that shadow tag takes a free turn to do its job, I would argue that M gengar is never difficult to switch in with the immunity to toxic and it ability as a spin-blocker.
One thing I must admit though, is that preparing against Mega Gengar takes surprisingly less efforts, but perish song and destiny bond was not exactly taken into account in the process. If baton pass is sufficient to make Blaziken a quick ban instead of debatable, having access to two extra variety is huge.
Some tests may have to be done to justify the ban, but I would predict that it will be banned eventually.
Ou by far because of bullet punch sczior and lucario with sd focus sash alakazam cam revenge it mega pinsir quick attack chesnaught mind game. And other. Legend boosted choice scarf and ditto. Choice scarf Latios can Ohko gengar mega with psychic. So much to counter. In ubers choice scarf darkrai gratin a and etc beat this so I think mega gengar should be ou because I can alway deal with it. Also big bad talonflame
All the thing you mentioned only occurs to offensive teams, stall teams pass by and say hi