He's balanced, in the fact that he lowers your phase out counter.
A C'tan is very good against special characters and other things.
Their weakness is only to persistant shooting, or anything with rending.
You can use them any time you want, as proven in battle, they're effective vs some armies, but weak vs others.
I still think a monolith is better, because a monolith can shoot every unit within 12 inches, and helps your warriors regenerate.