> they'll think about scenarios first, then code after they have everything accounted for.

I'm one of those people. But I find it hard to believe that a test suite can reflect everything I understand about a problem domain. Focusing too much on unit tests makes it difficult to find the general rules that govern the behavior of a system. Have too few unit tests, and they won't capture all the subtleties and corner cases. Have too many unit tests, and they won't fit in your head.