The highezt cost of unit tests is when they are refs tired with code. The value of a unit test is greatest when written before the implemantation. When refactored it is usually just made to pass by any means necessary to stay green without much/any thought. They are also brittle in these cases where refactoring the test can take meant times longer than the code it tests.

My advice is usually to do tdd for new dev have good coverage of full function tests and delete unit tests if the get in the way/lose value on refactoring. No one likes deleting tests but they must add value or its just more debt.