That kind of unit tests sounds great. I always reach for writing tests for eg. for library functions. Though I tend to skip when testing some app functionality which basically requires stubbing/mocking tons of services, and feeding them what output to expect on what input - in those cases, I don't find tests or TDD very helpful.

I find a lot of value in unit tests for isolated logic testing and in functional tests for end to end testing. Still that leaves a lot of code in a modern web application (like stackoverflow) where I find tests don't provide enough bang for buck.