I have used Grails since version 0.42 and I am a bit surprised to hear that the documentation has been incomplete. I have always found it very detailed, though sometimes lacking clarity as any framework documentation would have. Since the early days, the way you build Grails application has been almost identical to how you write and structure your code with the upcoming 4.0 release. Of course if you have used some plugins that has been unmaintained, that could have caused issues with upgrading. But otherwise I have had a great experience through Grails lifetime.

And if you really needed to get "out of Grails", there has never been an issue using the Spring MVC api or Spring Boot with Grails 3+