Two things I've figured out with pretty good certainty 1) on an existing codebase/hand written, even fable absolutely will "make a mess" if you do large changes at once, or don't check the output.

2) with detailed, WELL NUMBERED (Claude is remarkably good at following number references), multi-level design and build documentation Claude absolutely CAN produce moderately complex applications (crud with some lightly branching business workflows).

The corollary to (2) however is can you actually get that working code to be production grade, AND maintain it? that far I haven't got yet.