One issue with mocks is that you have to ensure that the way they are mocked accurately represents the actual behaviour of the thing being mocked, in production

One issue with mocks is that you have to ensure that the way they are mocked accurately represents the actual behaviour of the thing being mocked, in production. Otherwise your mocks may end up driving the system to a state that is never actually encountered in production, while the unit test still passes. When you run an integration test or a real build of the application, the actual system itself will fail to behave as expected. The mock implementations can diverge from the actual production behaviour.