It's usually some commercial programs that have issues.. I was going to try it and give a more specific answer, but Ubuntu does not let me run twm. I get a blank screen when I log in with twm selected.
Anyway I tried it in vnc4server, where it does work, though you have to start it with LANG=C. But now I can't find any programs that are not working.. I'll have to use it for a while.