It is very strong defensively! He is only extra vulnerable to dark-type moves, outside of certain very specific conditions! There are fighting-types that learn a move which allows them to strike ghosts, but they must use that successfully first, and Emmet's quite fast! He could get in some good hits while they set up!

[Of course it can be a viable strategy anyway in the right conditions - but it's complex! That's what makes it fun!]