The big thing that's missing for me is support for operating systems that aren't Linux or macOS. At work, Windows support is a must (I may strongly dislike it, but that's life).

The other blocker was historically the lack of threading, but now that seems to be implemented.

Yes, Crystal is pretty cool. Though I have found that it is hard to compile for different platforms. If you need speed and concurrency, giving jruby a shot is worth a try. It speeds up your code through both being fast and having 'real' concurrency.