I wrote SwingSet partly as a proof-of-concept, and partly to make the use of common, basic Swing components easier to do without having to use a WYSIWYG tool (e.g. Netbeans).
Monkeybars was being misrepresented as having a dependency on Netbeans. This was never true. Netbeans was used because it plus Monkeybars provided the easiest, fastest, and most maintainable way to create robust desktop apps using JRuby. But you could always just go straight to using Swing components and build your app in vi or Sublime Text (or whatever).
One issue I ran into was in subclassing some Swing classes. I don't have the details handy right now. but there's at least one item in SwingSet that started failing because of a mismatched subclass. (I think it was related to changes in both Java and JRuby.)
I've run into this in other cases of trying to use JRuby to wrap some Java objects (such as with the LeapMotion library).
I don't have any immediate plans to extend SwingSet, so my advice would be to write your own UI component wrappers as you find the need for them.