I’ve never understood engineers who complain about vague specs… if the spec was complete, it would be code and the job would be done already! Getting a 20 page spec delivered from upon high and mechanically translating it to code without any chance to send feedback up the chain sounds like… a compiler.
The demands are for functional requirements. Plenty to translate on the non functional side of things.
In my experience, the complaints are not about the specs and their vagueness. It's more about the political game to get them detailed. If you've not encountered the kind of organizational issues where getting an answer is like pulling teeth, you're kind of lucky.
Yes, I don't think a job where I am programmed by a product manager would be terribly interesting. I would move on to be the product manager if I found myself in such a role.
Probably why I haven't ended up in any.