It used to have a minimal set of things you could install but moved to a clean Ubuntu box that you can pretty much muck about with and install what you like.
For Rails, it is perfectly suited and very easy to spin up an Ubuntu VM and play.
I used Cloud9 before hand and though this was a while ago, I had a demo of an Express thing with MongoDB and about 15 mins before said demo it started falling apart.
I believe the uptime is pretty great, just a lasting impression of failing just at the wrong time.
Codio seems to have pivoted however to be more about offering educational institutions the ability to create courses etc. so though powerful, that is what the developers are focused on now
Occasionally the version number of things lag behind in Codio and that is my biggest gripe. That and PHP boxes not having mod_rewrite enabled out the box, but that is just me