I wrote before about unit tests and one benefit being increased developer confidence. On the flip-side of this, unit tests can lead to developer overconfidence. There is no perfect replacement for thorough integration tests in a realistic environment. Most of us (developers) at some point have probably scratched our heads and said, “well the unit tests passed.”
Unit test success in such a case is still beneficial. It helps us to identify where a problem is not. (Or, worse, it helps us to realize that we need better unit tests.)
Just a couple of thoughts.