There's pretty much nothing you can do in Objective-C that you can't in RubyMotion.
RubyMotion exposes 100% of the Objective-C runtime with virtually no performance penalty. They are very on top of new iOS releases (released full iOS 6 support within a week).
In fact, I think that the RubyMotion community adopts new iOS technologies faster since it's new and doesn't have a lot of legacy code.
There is a significant difference between APIs that are named/structured in a Ruby-like way, and APIs that are named according to Objective-C/Smalltalk conventions. This can be a cause of significant pain. (For example, while refactoring, you have to search for all the ways a method could be called.)
RubyMotion is doing the right things. Much of the result also depends on community. It's a bad sign when supposedly smart programmers disdain a technology or a set of tech conventions simply because it's different. Especially when that tech has a great track record. It's really weird when they disdain the very thing they're building on. I've met some RubyMotion programmers like this, however. I hope they're just an aberration.
RubyMotion exposes 100% of the Objective-C runtime with virtually no performance penalty. They are very on top of new iOS releases (released full iOS 6 support within a week).
In fact, I think that the RubyMotion community adopts new iOS technologies faster since it's new and doesn't have a lot of legacy code.