Kingdra Ex is good, but it has a few weaknesses. It has a good attatck for getting it set, but it has 10 hp less than the others, and an electric weakness. Now I know that it takes less damage from stage 2 ex,a nd has no weakness when you do 80, plus you'll get set faster; this is the only thing. If you have no energy in the discard, plus what would you run with it?
Another big problem: Alot of pokemon have fighting resistance, and that could mean big trouble for Kingdra.