Any problems caused by allowing druids to wildshape into swarms?

I'm aware that RAW they can't, which isn't the question, but what would be the problems caused if I allowed it anyway? They have a CR and (aside from swarm of Cranium Rats which I would probably disallow) they don't seem to have any disproportionately powerful abilities. Thoughts?

Edit: Would definitely rule "The mental exertion of controlling multiple bodies is such that they cannot be made to separate and must act cohesively as a swarm."