For the sake of discussion I'm going to post this Smeargle set.
Smeargle @ Focus Sash
Timid - Own Tempo
252 Speed, 252 HP, 4 Defence
This is almost guaranteed to pass a Shell Smash onto a more threatening sweeper (Kyurem-B, Mega Garchomp, whatever). Against most Pokemon you can simply Shell Smash while they bring you down to your sash, then Spore them and you've got a free turn to pass to a sweeper while they switch out. Magic Coat bounces back Taunt, Roar, etc. The only things that can reliably stop this thing from passing are Fake Out users, Breloom (thanks to its priority and Spore immunity), and some multi-hit move users.
When you pass +2/+2/+2 to Kyurem-B, therein lies the issue. Baton Pass teams are not broken, because a full chain Baton Pass team is a gimmick at best. However at the cost of one teamslot you can double one of your sweeper's offensive stats. In the case of something like Kyurem-B, that alone is GG right there, especially since you still have 4 teamslots to support him (Magnezone to remove Scizor, ResTalk Gothorita to remove Breloom and other Mach Punch users, etc.).
In my opinion Shell Smash + Baton Pass and possibly Speed Boost + Baton Pass should be banned.