> I find mocks and interaction-based tests (rather than simple input/output tests) are this way

It's fascinating, I agree with the rest of your comment wholeheartedly but am a heavy mock user. Much of the code I interact with has complex logic with well-defined and discrete interacting objects. Without mocks, the combinatorial explosion of input/output behavior means tests are extremely difficult to write, or more likely, are written incompletely, without properly enumerating the invariants expected of the interfaces under test.

I'm currently working on a complex system right that's safety-critical but also SOTA, and full of domain experts who are decisively not engineering experts (among other things, engineering expertise is part of what I bring to the table). Well-defined invariants and narrow unit tests that enforce them are a _god-send_, and I'm literally not sure we could function without them: as we're constantly refactoring the system to work in different ways, automated testing of invariants enables a whole universe of experimental changes that would otherwise carry a great risk of introducing bugs into other parts of the system. Our system is far too complex for integration tests to cover a fraction of the completeness that unit testing enables (though we also have simulation/integration tests, of course).