Well, I cannot really explain why because I lack years of experience with Java and Plug-in Development Environment, but the problem seems to be that my XML unmarshalling is in a different plug-in as my generated classes.

If I create my JAXBContext from ObjectFactory.class in a plug-in project different from where the package nodeset.generated is defined (package that holds the XJC generated classes), then the context doesn't know about UANodeSet class. I find it strange, because nodeset.generated is being exported, and imported in the unmarshalling package.

I have moved the unmarshalling methods to nodeset.generated and everything worked like charm.

If someone can give the details on why this could be, I will happily edit this answer.