JRuby is a very mature and widely used implementation by now. Granted, it's a bit isolated from the world of MRI libraries with compiled extensions, but compatibility story is getting better with every day. Besides, you are within arm's reach from the java library world.

That's the point though, as sad as it is, it's not "there" if it's not drop-in, given the stranglehold MRI has on the Ruby world.

You can run most applications with JRuby but to my knowledge, it always involves fiddling and tweaking.

For a Rails application, using the jdbc adapter or Puma/Trinidad instead of the regular servers for example. I am not saying this is a proper justification but it does prevent people from testing it further.

Also, I am still looking for actual and undeniable proof that using JRuby instead of MRI does actually bring better scaling/performance/memory management/tooling. So far, the examples I have seen are conclusive, but still limited to very particular use cases. That Heroku now officially supports JRuby is a big plus I would say.