Sure but the problem is they are slow and the combinatorial explosion of possibilities at the edge of your application will kill you. You can't enumerate every

Sure but the problem is they are slow and the combinatorial explosion of possibilities at the edge of your application will kill you. You can't enumerate every possible code path in an application of reasonable size. This is why a balanced approach is usually employed: a small number of representative happy/unhappy path integration tests to test the high level user feedback etc, with lower-level tests on the underlying layers and libraries etc. If you could just do it with high level tests, we'd all do it that way.