Well the parallelism issues doesn't exist in the jruby world, but yes, gems with C extensions normally don't work out of the box with jruby, but in another hand you can call java straight from ruby. That's so powerful and often better than the gems - if you know how to right ruby and wrap the java classes with ruby! So as many times said along this thread, ruby isn't rails. I have a project running in a government agency with jruby. It was a port of an old Java SWT application and we rewrote it in jrubyFX. The project was such a pleasure!