Shedinja is even more vulnerable to the always-present stealth rock, as well as poison and burns, than Cloyster is. Its attacks have less power and can't ignore substitute and sturdy. Shedinja is also slower, so things like Electrode, scarf Houndoom, mega Sceptile, Swellow, scarf Shaymin, and scarf Tyrantrum can revenge kill Shedinja but not Cloyster. Its 1 HP and lack of ice coverage means that Nidoqueen, Rhyperior, Mandibuzz, and Necrozma force Shedinja to switch, instead of the other way around, and Shedinja is less useful for decorating cakes. Cloyster gets both spikes and shell smash and can use either when forcing switches, making it much less predictable than Shedinja. Things like Machamp, mega Sceptile, Roserade, and most taunt users stop Shedinja from setting up even more easily than they stop Cloyster.
The other good shell smash user is Barbarnacle. Unlike both Shedinja and Cloyster, Barbarnacle has better defensive typing, so it can switch into stealth rock much more freely and set up on Drapion and Tyrantrum. Barbarnacle's higher physical attack and wider attacking movepool let it deal a big chunk of damage without setting up and sweep through steel opponents.
Shedinja will also likely be outclassed by Golisopod. Like Cloyster, Golisopod learns spikes. First impression and higher physical attack mean Golisopod is less reliant on setup and better at revenge killing things like Virizion and Barbarnacle. 1 HP makes Shedinja worse at checking mega Blastoise, Donphan, Zygarde, and Araquanid.
Maybe Shedinja can still force out just enough opponents to be viable, and I'm wrong.