I don't really say much on Smogon, but here's my opinion at least.
From playing a lot of Ubers in gens 4-6, there are obviously pokemon that are clearly more overcentralizing, so to speak, than others. That being said, Ubers wasn't treated as a tier as much as a banlist until some time around gen 5.
Now I keep seeing the arguments that nothing in Ubers should be banned because there will always be "overcentralizing things", or that since it's Ubers nothing should be banned. Those are not valid arguments, and said arguments reflect your knowledge and skill level about the tier. Pokes like Kyogre or EKiller Arceus are extremely versatile, but still have checks and counters.
Anyway, I know I shouldn't bring up Gothitelle when this is a Gengarite discussion before Shadow Tag as a whole. But at least in my opinion, Gothitelle, while a strong force, can really only do so much, and can often find itself as near deadweight against more offensive teams.
As oppose to Mega Gengar, who can fit on any team and do well. The reason for this is its versatility. Due to its movepool, one can make Mega Gengar run specific moves to remove most, if not all, of the biggest threats to your team, offensive or not. Other Shadow Tag users are etremely limited in their sets. A Shadow Tag user can be deemed useless due to team MU, but as I previously said, that's not a concern for a team if you just build Mega Gengar well. And even if you can't handle the opponent's team with it, Destiny Bond takes the icing on the cake. If it's useless, just go kamikaze, waiting for the right moment. In the meantime, you can dish out damage.
An argument I have been seeing is that you still have to mega evolve first, so you're able to switch into a check. If your moves, for some reason, doesn't cover the check, you can just switch out. Granted, other Shadow Tag users can too, but again other ST users are a lot more limited. Or say you sent out Gengar on a double down. If you know that the opponent's pokemon is not choiced, you are free to mega evolve. But if it is choiced, you can run protect to not only scout the locked in move, but also freely mega evolve to get ST, as well as your beefed up stats.
My points lead to my conclusion that while Shadow Tag is an incredibly powerful ability, Mega Gengar is over the top, being able to do everything your team dreams of wanting, something the other ST users cannot do. Gengarite should be banned, but Shadow Tag should stay.