Ruby and Objective-C
Ruby is very similar to Objective-C because both have a lot of similarities with Smalltalk. However, Ruby is a messy language on many levels. Many consider it a mostly a feature, not a bug, but it indeed makes interoperability with Objective-C less nice.
1. Ruby has a messy syntax. It inherits several features from Perl, Bash, Python and C-like languages and mix them together without any clean distinction. You have always many ways to do the same thing and it is never obvious which one is the best. With Objective-C it is much easier to write a lot of boring and obvious code than with Ruby. Of course, this property appeals to coders who love funny things in their code, but it does not play well with people who want to ship huge complex applications and systems in time.
2. Ruby has a messy standard library. Many things in standard library are not well-thought, not consistent with other things, redundant or lacking essential functionality. Things like File vs. FileUtils, Date/Time/DateTime + ActiveSupport extensions demonstrate horrible inconsistencies at some core things.
3. Ruby has a messy culture. Ruby coders *love* smart tricks and hate boring code. Everybody wants to DRY all the code for better or worse or pretend being a cool meta-programming lisp hacker (not the best professional quality in itself). People are not careful about containing their libraries as humble as possible. Many people are okay to inject methods into core classes and do a lot of meta programming tricks and be proud of it. All this leads to maintenance issues and is not compatible with Objective C culture.
These three key things make Ruby incompatible with Objective-C or Cocoa framework.
It’s important to add that all of this is not unique for Ruby, but pretty much for any hobby technology, particularly backed up by some open source community.