After much council and community deliberation, including a public thread, poll, and a Policy Review topic, the OU Council has come to the decision that Gengarite will be quickbanned from OU. Remember, a quickban is considered an extension of the initial banlist, meaning we are implying that Gengarite deserved to be on the initial banlist and is considered that overwhelmingly strong.
Mega Gengar is not a Pokemon that will easily sweep teams, in spite of its 170 Base Special Attack and 130 Base Speed (enough to outspeed almost every unboosted Pokemon), and its offensive prowess is not the sole reason for its quickbanning. It is a Pokemon that allows teams to, with near 100% probability, remove specific counters or threats and open the way for an easy victory. This capability is especially pronounced against defensive teams, and to some extent any kind of team relying on slow Pokemons, to the point that the Council believes that Mega Gengar is stifling the development of defensive teams overall. Note, we don't necessarily mean just stall teams (though these are affected significantly), but defensive teams in general. This overall detriment to defensive teams (remember, not just stall) is considered a pronounced negative for the metagame's development (hence why we're not waiting on PokeBank to ban it).
Shadow Tag allows Mega Gengar to trap and kill whatever Pokemon you want it to eliminate, generally the ones that give your team trouble or the ones that stop your sweeper. Defensively, Mega Gengar can easily switch in with its useful resistances and immunities to common types, and offensively, its high Speed and Special Attack stats and access to STAB Shadow Ball with Focus Blast as coverage allows it at least neutrally damage the entire metagame. In addition to its offensive and defensive prowess, Gengar has very useful support moves, such as Will-O-Wisp, Taunt, Destiny Bond, Hypnosis, and when PokeBank is legal, Perish Song. These moves give Gengar additional utility against both offensive (Will-O-Wisp and Destiny Bond) and defensive (Perish Song) teams, and Shadow Tag is the tool that allows Gengar to remove nearly all defensive threats or counters or checks with what the Council has determined is too high a probability, note that, while some Pokemon can handle Mega-Gengar in a one-vs-one scenario, it has no true counters due to the nature of Shadow Tag (aka: switching in your counter), and that fact alone is the main reason as to why we thought it deserved to be banned.
While it's true that Shadow Tag will not be in effect the turn Gengar mega-evolves, which allows the other player to send in a counter or a Pursuiter, Mega Gengar can still Substitute in the same turn it mega-evolves, put opponents to sleep with Hypnosis, or Destiny Bond, and then switch out without the risk of being Pursuited (or in Destiny Bond's case, at least remove the Pursuiter.)
Mega Gengar also has a huge movepool. Depending on what its team needs, it can be a hard hitter to destroy something that could give the team trouble (for example, it can kill fairies with Sludge Bomb, kill Latios or Latias with Shadow Ball, and kill bulky waters with Thunderbolt, etc), burn physical attackers with Will-o-Wisp (destroying a lot of Pursuiters as well), put to sleep any opponent it wants, use a Perish trap combination, Destiny Bond sweepers, Toxic walls, or recover HP with pain split. While we do recognize that Mega Gengar obviously cannot do all of this in the same set, we are mentioning all of what Mega Gengar can do offensively, defensively, and supportively to emphasize that those tools, along with Shadow Tag, create a Pokemon with the potential to be far too effective of a support Pokemon.
Though we aren't directly using the 4th generation suspect characteristics anymore, they still provide a reliable framework to assess the situation. Consider the Support Characteristic:
A Pokemon is uber if, in common battle conditions, it can consistently set up a situation in which it makes it substantially easier for other Pokemon to sweep.
Mega Gengar is the perfect example of this. It is not broken solely because of its offensive stats. It is broken because the combination of Shadow Tag, its great defensive typing, its great offensive stats, and its huge movepool allows you to, with almost absolute certainty, remove one or multiple checks to your team with Mega Gengar. We feel that this near 100% probability against all Pokemon (except Ghost-types) is uncompetitive, as Pokemon battles are (and for the most part, should be) a 2-way street in terms of decisions; this Pokemon makes the battle a 1-way street against nearly all Pokemon.