I agree upgrading Grails has been painful in the past(pre 3.0), but more for underlying Spring/dependency upgrades/changes. Most of the upgrade pains I’ve seen from Grails itself have been from people using bad practices

I can think of counter examples. For example in Grails 2.5 the various

*Holder

classes were removed. This change was made within Grails itself, i.e. it was not necessitated by a Grails dependency. This was a breaking change in a point release, which makes the job of upgrading an app from 2.X to 2.5 a nightmare, because most of your plugins won’t work, and there won’t necessarily be a 2.5-compatible version available.