One thing that I have observed is, there is often a problem dependent sweet spot for the integration level of a test suite. Sometimes that is literally testing the behaviors of every method while other times it can be end-to-end testing or somewhere in-between. The challenge is, it can take a great deal of thought to arrive at the appropriate inflection points. One approach I take is to try to think about what suite of tests would make it easier for a developer who is new to the code base to be productive in it. They should feel that the test suite is helping them, not holding them back.