Well, for TDD specifically I tend to think of it as a testing methodology for libraries. Application logic tends to be hard to test that way since they're about

Well, for TDD specifically I tend to think of it as a testing methodology for libraries. Application logic tends to be hard to test that way since they're about human interaction. Of course, you want applications built out of libraries. But eventually if you do that you end up at the point of "this is pure UI code and unit tests will do me no good." So I don't think it's just "stable, mature product" vs. "new product". It's also about code structure and where the abstraction layers are, and which abstraction layers are stable and therefore amenable to automated tests. A UI can change in ways that a human will not even notice but will break brittle automated tests. There's workarounds, of course, and probably it's possible to build much better tooling than actually exists.