I almost only use mocks for external calls (usually external APIs). Very rarely, I use them for some internal code that is unusually time consuming or difficult

I almost only use mocks for external calls (usually external APIs). Very rarely, I use them for some internal code that is unusually time consuming or difficult to set up. A few months ago, I did a contracting job and the team I worked with used the "mock everything" approach, without even one integration test, which to me seemed crazy (especially for a component which was calle "integration layer"). I tried hard to find the advantages in this approach, studying what the rationale and the best practices were, and questioned my previous assumptions. In the end, I had to confirm those assumptions: even if there were hundreds of tests and they all passed, many logical errors weren't caught. Even worse, they gave a false sense of confidence to the team, and made refactoring super-slow. But the team leader was super-convinced it was the best idea since sliced bread.