Why would it be of concern? I think mocks have a good place in unit tests. I generally believe the premise of unit tests is to test your code against specific pre-conditions. If those pre-conditions cannot be created in a unit testing environment then mocks are justified. I would, however, be very way of mocks moving past unit testing and into integration or system testing (or other varieties like failure injection testing, performance testing, etc.). Mocks have their uses & their limitations. Understanding that is key to using them well.