I tried JavaFX circa 2009 and it was scary; pure black magic in terms of what the same code would on a given day. I guess Oracle put some effort into it over the years, but never, ever again will I touch that language.
It's not a custom language anymore. It's just a regular widget toolkit API like Swing. There is a thing called FXML which is a run of the mill XML based layout language, with a GUI designer called Scene Builder.