I think a lot of Rubyists unfortunately don't write thread safe code :( I've worked on many Rails apps that have some mutable state on the ApplicationController and the models. Also, developing on the JVM requires a change in tooling that a lot of Rubyists aren't used to, as well as the slow JVM start up times which can make TDD with JRuby a pain. JRuby is consistently getting better, though.
> mutable state on the ApplicationController and the models
Well, mutable state on the models seems pretty fair, unless I'm misunderstanding you. It's rather unforgivable on ApplicationController though.
Edit: and the slow startup time doing TDD with JRuby was my primary complaint when I developed in it. Although iirc you can use nailgun or something to keep the JVM active and mitigate that pain.
Is the JRuby environment sufficiently different that you can't TDD against MRI and then save JRuby for final validation testing and deployment? They shouldn't be that different, unless you're not writing pure ruby code.
Alternatively, Nailgun should reduce a lot of that pain. Effectively you have a running server instance of JVM and run some clients on it. That gives you JVM speed advantages without the slow startup. JRuby natively supports it: https://github.com/jruby/jruby/wiki/JRubyWithNailgun
