Mence has two particularly viable sets, Dragon Dance and Mixed. The problem is that while they both have decent checks, the checks aren't the same for both. So if you switch to, like, Hippowdon or something expecting the DD set, and eat a Draco Meteor instead, you're screwed. Obviously there are ways to play around it but the point is that Mence 2HKOs everything in the game except like Cresselia, and Cresselia kind of sucks to be honest with all the Scizor and Ttar running around.