More specifically regarding spec-driven development:

There's a good reason that most successful examples of those tools like openspec are to-do apps etc. As soon as the project grows to 'relevant' size of complexity, maintaining specs is just as hard as whatever other methodology offers. Also from my brief attempts - similar to human based coding, we actually do quite well with incomplete specs. So do agents, but they'll shrug at all the implicit things much more than humans do. So you'll see more flip-flopped things you did not specify, and if you nail everything down hard, the specs get unwieldy - large and overly detailed.