I was evaluating Griffon as a framework. I got the impression that this project is slowly dieing. IMHO Groovy is not a mainstream anymore (I wonder if it ever was a mainstream?). Now everybody fancies Scala.
Now back to your question:
- Most frameworks expect that you follow the standard development path. Any changes / customizations will most probably introduce difficult to maintain solutions (they will call it architecture afterwards). Choose a framework that allows you to do 95% of the things you plan to do. And yeah, choose a mainstream framework.
- Griffon is based on Groovy, so you have to master Groovy first. Ok, Groovy is a JVM language and if you're OK with Java it will greatly help, but still all those DSLs will require some time to settle in your head.
- If you know any mainstream framework - this is a valuable asset. The sad fact is that frameworks tend to fade / die and you have to constantly look for new buzz things. You never stop learning (although key principles cannot be changed and remain constant from framework to framework)