The single reason that makes Gothithelle so dangerous (and a suspect) is being a Shadow Tag user that can act offensively. Some of the arguments that have been brought up are ''it's too slow'' or ''it's defenses are just average''. It doesn't really matter, since Gothithelle is fast enough to outspeed what it needs to -basically, walls like Gligar, Qwilfish Rhyperior, Slowbro or Snorlax, which Gothithelle can either outright kill (or Trick / weaken with Psyshock in the case of Snorlax), meaning pokemon like Mienshao, Heracross, Raikou or Choice Scarf Darmanitan have a much easier plowing through teams, as a simple U-turn or a double switch to Gothithelle is enough to get their counters out of the way. While Gothithelle can only kill 1-2 mons maximum per match (which would be an argument against its ban), that's almost always the only help some strong sweepers need to be able to sweep. Of course, Gothithelle is usually much worse against offensive teams, but it has the defenses to at least revenge kill 1 pokemon of its choice, something that can be crucial in the course of a match since offensive teams usually rely on Choice Scarf users to deal with some threats. I'm not totally convinced that Gothithelle is broken, but I think its going to be a bad influence on the metagame.