As somebody not into the whole ruby-ecosystems: What's an example that breaks compatibility between ruby and other implementations? And what's so wrong with JRuby that you won't even consider it?
I have considered it deeply. Many of our enterprise systems are Java-based, so JRuby's ability to call Java methods directly without implanting services would be a huge pragmatic boon. But every time I consider that I have to balance it against being able to use a constellation of gems in Rails. For example, nokogiri (libxml) is used in a ton of stuff. Sure there are Java equivalents, but that isn't the issue. Rather I would have to reimplement or find replacements for everything we use that depends on that without introducing side-effects. In my experience that's really hard unless it's a toy application without many dependencies. (Read: opposite of most enterprise Ruby apps)
Also, as a gem author I've tried to support jruby along with MRI, but it requires jumping through a lot more hoops. Of course any cross-platform code requires more work to support, but it's a pressure on small devs. If no one needs it, no one helps support it, so then it becomes another gem that is effectively MRI-only. Kind of a viscous cycle.
Rubygems forms an ecosystem of libraries on top of Ruby. Most gems are dev/tested within MRI, some work across other Rubies, some do not. It's difficult to tell which is which. This means most teams choose MRI in much the same way that most Ruby teams choose some form of unix -- compatibility.
Ruby also suffers from the lack of a formal VM, so other implementations are merely similar rather than being guaranteed to run all programs. This is not the same with JVM vendors, where choice of IBM vs Oracle is largely based on extrinsic features rather than intrinsic compatibility. (Or at least you have to get in really deep to find differences-- in Rails you usually find incompatibilities that stop you from even starting the app).
A different driver to the database for MRI and JRuby. Not a big deal.
The deal breaker for me is the very slow startup time for tests and no gains for run once scripts. I've seen people developing with MRI and doing CI on JRuby to work around that. It's worth doing that only if you have a service running with some real load. Most internal services do nothing almost all the time.
