I imagine Shumway will never really be usable compared to the outdated linux flash plugin provided by Adobe. That's ok though, because hopefully by the time Shumway catches up, we won't even need flash players!
I hope this never reaches official release status. Flash MUST die and all the plugins with it.
While pdf.js has got a lot better since I first tried it (I still try it once in a while to see how it's doing) it still can't beat a proper document viewer written in a sensible language.