We test the shit out of our Apis. We do more API level/integration testing though.

I.e. a test will be something like "if the db is in this state, and we hit this endpoint with these params, does it return what we expect and update the db correctly".

Our app is primarily about users maintaining stuff on big datasets with complicated aggregation and approval logic. So setting up a scenario and checking the app does what the business logic says it will do is what we want to know.

It makes refactoring wayyyyy less painful to just know that the app will always behave itself. Rather than testing whether a function can add 1 + 2 correctly, we can test each endpoint does what it's supposed to do.

It gives us loads of confidence that the backend is doing what it's supposed to. If you do a huge refactor you don't need to worry about whether you broke the test or if the test is failing correctly. If the tests all pass everything is working as it should.

Downside is longer test execution times (because a temporary db needs set up) when running the full suite. Worth the trade off for us though.