Can't say I've used Shaymin competitively, but I have had one used against me. In my novice days, I was taken by surprise by an unusual Choice Scarf Shaymin. Turns out, it can revenge kill somewhat well. Said Shaymin had good synergy with Aerodactyl and Nidoqueen, so beating it wasn't easy. Moral of the story: if you're running hyper offense in UU, be prepared for a Shaymin that can outspeed most of your unboosted team. Choice Scarf is an OO, but it's not a bad one.
The risk I took was calculated, but man, am I bad at math.