Except Cloyster sets up on more offensive mons than breloom.. just FYI it can set up on garchomp, gliscor, mamoswine, choice locked physical attacks, especially choiced ice attacks used to revenge its dragon partners (if you aint using cloyster without least 1 dragon, you're doing it wrong). Breloom can pretty much only set up on slower or bulky mons that can't ohko it..
Also FYI cloyster gets ice shard to surprise breloom and future scarfed serperior.. you don't have to run razor shell if you're using kings rock as your item... 40% flinch rate isn't easy to mess around with and will help you break past common counters such as Keldeo more often than you would think. Clutch 10% flinch on ice shard can be so funny as well, but its only happened once to me as I got a final hit in before a scizor BP'd and then the next shard koed!
I've had great success everytime I used cloyster, it's somewhere between A and B imo, but more towards B.