One problem is that Graal is still on Java 8 (nearly two versions behind of latest stable) because it is marked as LTS. I think that's a bad decision for a bleeding edge piece of technology anyway - a lot of of the components that Graal needs are batteries-included in later Java releases.

And most importantly, there's this - https://news.ycombinator.com/item?id=16919776 ( Java SE 8 business users must buy a licence from Jan 2019 to receive updates)