I think we all knew it'd come down to this eventually, even if we didn't know when it would happen or what would be targeted - we now have a suspect test ON THE UBER LADDER. The target? Not Arceus, not Xerneas, not Kyogre - no, it's Mega Gengar. Indeed, just packing it is usually synonymous with getting one KO out of it, allowing powerful sweepers to rip through the rest of the team now that a crucial hole has been punched into it. I think everyone realizes that had it been any tier but ubers, Shadow Tag as a whole would be very much deserving of a ban (and yes, even Gothitelle can have an easy time tearing through shit, before you ask). However, there's the ethical issue of ubers being where all the overpowered shit goes anyway - including Mega Gengar. So there are huge disagreements on whether ubers are a banlist or a full-fledged metagame. And it's not hard to see why - the uber list is, at its core, a list of Pokémon that break OU and are therefore banned from it. Ergo, a banlist. But at the same time, any ladder whose core element is allowing these Pokémon is also a metagame. Furthermore, ubers are the third most popular ladder on Smogon, behind OU and UU, so clearly it's a popular metagame to boot.
With that said, why Mega Gengar instead of Arceus, Xerneas or Kyogre? Well, those three can be checked, for one. It's not easy, hence why they're so popular to start with, but it can be done. Mega Gengar just comes in, Taunts, Destiny Bonds, kills, GG. Stall suffers enormously as a result, and it's enough to invalidate an entire playstyle.
It goes without saying that there are other things currently banned from ubers, like OHKOs, Moody and Swagger. These, however, were banned due to their random, no-skill nature. Getting rid of Mega Gengar, though, is something that many people fear may take us down a path where they don't want to go. It'll be interesting to see how this goes.