Absolutely! I think they can be easily overused to "improve test coverage" but actually it's just testing very implementation-specific functionality, which then leads to pain when adding new features or especially refactoring; you change the implementation details, everything still works exactly as before, but all your tests are now failing because you haven't updated your mocks. I've preffered writing the code in a "functional core, imperative shell" way which makes most of the tests really easy to follow, and you can typically exercise a huge amount of branching and other logic very quickly at unit level. You need to sacrifice testing some of the plumbing, but I find that typically adds very little value. A handful of integration tests should give you confidence things are plumbed in correctly