Testing against real services (probably in some kind of staging environment) is mandatory, IMO, but it is only really useful for verifying the happy path, or so

Testing against real services (probably in some kind of staging environment) is mandatory, IMO, but it is only really useful for verifying the happy path, or some simple failure modes. Unit tests can verify the system's logic is correct in the face of complex edge cases that are difficult to replicate in integration or end-to-end testing. Unit tests also allow you to add new functionality to the codebase with confidence, as you can know that your modifications did not break some specific edge case or cause some bug regression. If you're in a dynamic language they also help ensure your refactoring did not introduce some type mismatch somewhere. Which is why I draw the conclusion that anyone who thinks unit testing and mocking boils down to `response = "derp"; assert response == "derp"`, must not have much experience working in such codebases.