Some people have an aversion to "goto" statements, and Java annotations are even worse, they're a "comefrom" statement, where you end up executing code before or after your function based on this annotation, so it makes the code really annoying to follow.
Java examples which show trivial code annotated with @POST or @Path are not representative of production systems, where you may have a lot more annotations for your DOM, documentation, and in some cases, you actually have more annotation boilerplate than you have code in the handler/controller.
Having annotations interleaved within your logic makes it difficult to provide good API documentation, and it's hard to automatically refactor, because your boilerplate is interleaved with real code. With an approach like this grumpyrest, you can put all your machine generated code into a package, and simply connect it to your hand written code with a little bit of glue. It makes spec-driven development much easier.
OpenAPI is very popular, and annotation based frameworks make it more difficult to integrate with it. If you generate API docs automatically from code, as with JAX-RS, it's easy to break things by accident because nobody audits machine generated docs. If you reverse the approach, and do spec-driven development, you code review the API behavior, and the code follows, which is a better model, in my opinion. Grumpyrest looks like it makes integration with spec-driven workflows quite easy.
A word of caution to the author; if this takes off, you will be inundated with issues and PR's, since people will use this in ways you never dreamed of. I'm experiencing that kind of onslaught in something I open-sourced for Go for REST API's.