very common

There are teams on high ladder completely riddled with Shell Smash abusers. Obstagoon is a common and predictable one, but I've also seen Tyranitar, Reshiram, Eternatus, Black Kyurem, GDZ, and more. I know fwqef posted a team who has multiple Shell Smash abusers and I've seen people reach the peak of the ladder with very minimal effort abusing mons with Sash and Smash. These Pokemon can completely trash teams who aren't prepared with multiplesetup checks. Obstagoon is used because of its immunity to Spectral Thief. There are Pokemon who are disguised as breakers (I've seen Shell Smash Intrepid Sword GDZ) and walls (Simple Smash Eternatus on a team that already looks highly offensive). All of these Pokemon get free turns to set up unless you can somehow predict them ahead of time. It is possible to overrun a well built team just by setting up and breaking a single mon at a time. Beat Obstagoon? They'll send out another Shell Smash mon and then you gotta deal with that.I know I haven't been horribly keen on the issue in Gen 7, but I know Shell Smash had popular support for a ban then. It's worse in Gen 8 specifically because of there being too few good walls, and offensive checks being limited. Galvanize is completely unused in favor of Bolt Beak spam. Refrigerate Black Kyurem is mediocre, but I haven't seen it used specifically to check Shell Smash sweepers. I've seen it more or less used as a suicide breaker by using Explosion to cripple annoying walls. Aerilate is completely unused due to the lack of viable Flying-types. Pixilate Zacian sucks. Triage exists as offensive priority but it's either pretty much entirely unexplored, or unused for good reason. Not to mention Dazzling exists.As for defensive checks, Unaware is ass right now due to the only particularly bulky Pokemon stat-wise being Eternatus, and it wasn't even terribly great in Gen 7 either. Power Trip, Stored Power, Sunsteel Strike, Moongeist Beam, and Photon Geyser all render Unaware useless. Anyone who can build remotely well can reduce the effectiveness of Imposter, so I would not say Imposter is a clean check to all of these teams, and even then you gotta have Scarf on Imposter if you don't want 50/50s. That largely leaves Prankster which is a solid means of beating these setup abusers, but you can't switch it into a boosted mon which brings forth the issue of Shell Smashers being able to take out one mon at a time only for another one to come on out and set up again.I should put emphasis on the fact thatHigh ladder is riddled with Shell Smash abusers, sometimes with people running three abusers on a team. I've seen people base their entire offense on setup. And seeing a majority of my high ladder games recently be consistent of different accounts using the same general team structure (multiple shell smash users, some of which are lures), I've come to the mindset that Shell Smash should be Suspect Tested.