Unit cost and quality.

First, get that "Steal" spell, and then we'll see. Haven can also get Exorcism or Magic Bolt. You can come up with a lot of ideas...
And the Nagas are good units. A good counter. But they could use a speed boost, which Haven doesn't have. Unless they steal it from the enemy (Nature, Chaos)...
P.S. And I think they can't attack everything... after all...