It really depends on the annotation processor and what kind of code you are trying to generate. In the author's case it shouldn't take that long nor would it be hard to develop and debug.
Also I've worked places with this magic and no induction or documentation or time for anyone to explain anything. The more behind the scenes hard to discover stuff you implement the better the docs and onboarding needs to be. It can't just be done to fulfill a architect astronaut fantasy.
I don't care so much about the documentation, as long as it's part of the build script (it's own documentation) and not checked into source control. By not checking it into source control it won't be edited, there won't be a mix of generated and custom code. By being part of the build process you avoid the generation tool and scripts being lost to the sands of time.
I'd love one that I could just point to a build tool/script instead of creating it's own.