When it comes to community support, I've found Hibernate to be one of the most frustrating open source projects to work with. It was comical how often I'd search the forums, find a person with the exact same question that I had, and see that Gavin or another project member had replied "you shouldn't want to do that" instead of actually answering the question.
Also, a couple of my projects were bit pretty hard when they removed a documented feature (it was in the manual!) in a point release (3.2.2 to 3.2.3, I believe), where the only workaround ended up requiring a significant refactor of our code base. The official response (ticket HHH-2667):
> I dont know how many times i need to close cases with almost the same exact description.
> This was never supported syntax, and was never supposed to work. It was simply a regression that it happened to work (somewhat) in certain releases.
So, even if Ceylon had considerable advantages over Mirah or Scala (which doesn't seem to be the case), I'd probably avoid it for the above reasons alone.