The only gems that do not work are those which are not pure ruby. For the popular gems I think there have been rewrites. I haven't looked into JRuby but it has changed quite a bit in the last year from what I read
I forgot about thread safety in pure ruby gems but that is not a huge problem - https://github.com/jruby/jruby/wiki/Gems-known-not-to-be-thr...