"The test suite needs to be fast. It needs to have decent coverage, but doesn't necessarily have to be perfect."

Holy hell, what a telling statement that is. I get not unit testing for 1 == 1, but come on, unit and integration tests for, say, user login should be difficult, not fast. There are some test suites that actually do need to be perfect, unless Instagram thinks that eg OWASP isn't "decent coverage".