Cloyster's downfall also comes in the form of poor type coverage.
His strongest attacks are Rock and Ice, which together don't offer much, bar completely breaking flying types, and he must also sacrifice a move slot for his new boost, meaning he cant carry all of his utility such as Rapid Spin and Explosion in addition to his offensive abilities.
Cloyster also gets completely broken by Priority.
Lucario, Infernape and Breloom, 3 extremely dangerous offensive threats, all have super effective Priority moves which are far more danagerous after cloyster lowers his own defense, not to mention that two of those resist Cloyster's own Priority attack, should he run it.