Depends on the target. It would be much harder to implement Java with a assembly target and some of the standard library because you would have to implement that from scratch. Not only that, but you have to deal with vtables from inheritance and class layouts. C doesn't require any of that complexity.

That being said, I think it would be very valuable to see somebody implement at least a partial version of Java with standard library (such as generics or at least collections) with assembly as the target.