Personally I have learned that TDD and unit tests are only useful in OOP settings that have side effects and have mutated state overtime. But with a more FP app

Personally I have learned that TDD and unit tests are only useful in OOP settings that have side effects and have mutated state overtime. But with a more FP approach all you need to do is verify the feature works since you know that state can't change the output. I kind of also feel that TDD has a kind of cult like following, especially with the Rails crowd