It is still now distant enough that I can't recall specific examples, but what I had found is that the solutions in blog posts etc. on Grails 2 (large majority of search results) did not apply to or did not work with Grails 3 when we tried them, unsurprising given a major version change, but frustrating nonetheless. I suspect that since you were following Grails's progression from such an early stage, any incompleteness or lack of clarity in the documentation may have not been so obvious to you because you would have been able to implicitly fill in the gaps, but we as newcomers found it totally inadequate. It was often reduced to black-box trial and error.