I believe that reliability is vital to the success of a Yawn lead. The worst thing that can happen is for your lead to faint before it can do anything. Leafeon has so many weaknesses that I can see that happening far to often. Because of its merely good Speed and mediocre SpDef, you are likely to find yourself trying to win battles with an unboosted Lucario and Slowbro. There are several other issues, but I've probably already posted some of them elsewhere in this thread. Hopefully this was helpful. BTW, I really like your avatar.