I think I figured this out. When I first tried to launch, I didn't have a directory of working TTF files in /usr/share/ (I did have some but they seem to be broken). According to the top answer here, that can cause a problem with JavaFX. With that directory, and with at least one TTF font inside it, WU launches.
Incidentally the developer in that post suggests OTF files should work, but none of mine do.