Honestly though, Roserade isn't much of a threat unless it runs the Spikes set. The Sleep Powder set is easy to counter because most of it has Toxic Spikes and I haven't seen a very successful Toxic Spikes team succeed yet. Even when I ran the Standard LO set, I wouldn't dead switch into Roserade or yet alone face against it unless I know Sludge Bomb/HP Fire doesn't OHKO me. Even if that Sludge Bomb does around 70% with max Sp. Atk, if Shaymin is able to net an OHKO, it just nailed a general big threat. Yea, I agree Roserade is a threat. But your trading off the safer ability to stop one threat to do other jobs way better.Im still not sure, the big loss is Roserade tbh. Roserade completely walks all over that Shaymin, Shaymin never even has a chance. Roserade is also the 3rd most used Pokemon in UU.
It wouldn't be hard to switch in it as well because of the extra bulk it can take from a Surf or Iron Head, EQ, etc. With the EVs, it can survive a DD Ice Punch from Feraligatar, CB Ice Punch from Azumarill or any other users, takes Ice Beams better. Milotic Ice Beam 3OHKOs Shaymin if it doesn't have LO.
Edit: Again, I find that this is a superior option when playing with a team with lots of Bulk. It is also more reassuring to switch into more hits.