I agree and I think this is why OP said to be careful -- there are so many engineers out there who do not separate the ideas of code and functionality. When these people write unit tests, they are essentially just checking that the unit behaves exactly as it currently does (white-box style), instead of checking that the unit behaves as desired (black-box style). Every subsequent change to the codebase requires a corresponding change to the tests, even if the external behavior of the unit remains the same.