It's easy to write "unit tests" that use JUnit and some mocking library. They may produce code coverage that keep some stakeholders happy, even though the tests may not even be unit tests and the test may provide questionable value. It can also be very easy to write unit tests that are (in theory) units test but are far more complex than the underlying code and hence just add to the total software entropy.
This particular type of software entropy has the unpleasant characteristic of making it even harder for the underlying software to be restructured or to surface new requirements. It is like the test has a negative value.