my vote: do not ban Gengarite
my reasons(s)
ultimately I'm against the ban because it would cause an entire pokemon form to be removed from anywhere in the meta. No matter how broken it is I strongly believe that ubers is the place for broken mons and nothing that isn't chance based (like moody, swagger, OHKO, etc) is "too overpowered" for ubers. I'm not saying mega Gengar isn't incredibly powerful, but it's not unstoppable. It loses to any scarfed mon, meaning that if your oponent predicts your switch you might have to choose between your gengar being Ko'd and the pokemon it was trying to clear a path for being KO'd. (+1 iron head from scarf genesect OHKOs both mega gengar and xerneas). But even that being said, many people still claim that it limits the team building process (people also say the same for Xerneas but that's a discussion for another day) and therefore should be banned, but I've often found the opposite to be true. The fact is that ubers right now is laced with setup mons (Geomancy Xerneas, E-Killer Arceus, SD/rock polish Groudon, calm mind kyogre, PUP mega kang, the list goes on) so now more than ever phasing becomes essential to team building. Now if it weren't for taunt, mega gengar would hardly be a problem for these mons, since shadow tag does not stop yourself from being roared out, but taunt of course will stop roar in its tracks. Here's where I had the realization that made me definitively vote against the ban. See, shadow tag is definitely "uncompetitive" in the sense that it stops your opponent from switching and therefore limits their ability to predict, arguably the most interesting and competitive aspect of the game, but when going against any one of the ever so common phasing pokemon (groudon, palkia, arceus, etc) predictions, and therefore competitiveness come back into play. say your mega gengar is in againt an arceus steel, a pokemon that would easily wall your Xerneas and phase it out especially since you carry hidden power fire instead of focus blast, and on top of that your opponent has kyogre which will make the HP fire damage that much more pitiful. Therefore, knocking out this arceus is instrumental in shutting down the defensive side of their team and allowing a geomancy sweep; if it's gone then nothing can stop you. But there is a problem, you already know three of the arceus' moves having seen them early in the match: calm mind, judgement, and roar (chances are the last move is recover but that's asdie the point). It is at plus one special attack so a single judgement will knock out Gengar after stealth rocks (which are set up) so the obvious choice seems to be destiny bond. But if you predict wrong gengar will be roared out, so is taunt the best choice? Well the safe play is definitely destiny bond but either way this requires prediction on both players' part and therefore cannot be uncompetitive. Now of course the situation leans in the direction of gengar either way, but skill is still involved so Gengar is not "ruining" the match. Now of course this is very situational but anyone who has played a significant amount of ubers, and certainly anyone who has made reqs will be able to think of a situation where even if mega gengar is in play, a single prediction could mean the end of gengar without the end of your opponent.
Sorry for the long winded answer but I feel this is a very important issue and mega gengar is often dismissed as uncompetitive simply because it is difficult (but not impossible) to play around even without specifically preparing for it.