> A good way to understand if your unit tests are good: are you changing them along with changing your actual code? Then it’s a bad test. I think the argument for “it’s just documentation” is weak.

Unit tests are great for pure algorithms, like file format, data encoding, crypto,… etc. Everything with a specs that will rarely changes. You write your tests once and basically never have to update them.

But for requirements that changes often like in a enterprise settings or applications, maintaining a suite of unit tests is expensive. Integration tests are better because contracts between modules don’t change that much. Even if the suite are not exhaustive, they’re useful enough to catch some failures.