I can see cases like the recently mentioned pg_textsearch (https://news.ycombinator.com/item?id=47589856) being perfect cases for this kind of development style succeeding - where you have the clear test cases, benchmarks, etc you can meet.

Though for greenfield development, writing the test cases (like the spec) is equally as hard, if not harder than writing the code.

I also observe that LLMs tend to find themselves trapped in local minima. Once the codebase architecture has been solidified, very rarely will it consider larger refactors. In some ways - very similar to overfitting in ML