This isn't the first time something has been banned from Ubers. Moody is a prime example of something uncompetitive that was banned from Ubers last gen. Uber has shifted from being "just a banlist" to a tier, and while it is not a tier that aims to maintain balance, things that are uncompetitive by nature are banned.
Mega Gengar is one of those things. With the moveset of Perish Song, Protect, Substitute and Destiny Bond, it chooses what it kills, and usually nets at least 2 kills per game. Perish Song, Protect, Substitute, switch, bam, they're dead. You can repeat this cycle again if you're healthy enough, and if not, just Destiny Bond for your second kill. They only have that initial turn that you use Perish Song to kill you unless they have Pursuit, in which case you've still killed them in exchange for losing M-Gengar as well.
But this isn't all M-Gengar can do. It has the movepool and power to go offensive, which means bringing in a Pursuit user like Tyranitar on the turn it Mega Evolves to remove the problem as early as possible can be met with a swift Focus Blast, easily 2HKOing.
I know it's a hard thing to accept at first, and it's still very hotly debated, but banning this from Ubers was the only way to make Ubers a metagame worth playing. and since the tier is no longer just a banlist (since last generation), it was the only course of action to take.
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