I've only recently gotten into iOS development and the state of XCode amazes me. It's awful. Frequent crashes, disappearing tests, syntax highlight shutting down... it really makes me wish there were an alternative.
You can buy AppCode. It's based on IntelliJ, so if you are used to taht it's great. But it's not perfect and I usually have both IDEs open when working on iOS (in aprticular, running and debugging are nicer from XCode. I write my code in AppCode and run it from XCode).
There's nothing stopping you from using one. The great thing about OSX is that you can get to the command line just like Linux and use Emacs or Vim. Or even GUI IDEs like Geany if that's what you want.
A lot of this is worse for Swift than it is for Objective-C. Still, I do find it weird that such a mature product (roots going back decades, really) is still so shaky in fundamental areas. In many ways it has actually regressed since e.g. version 3.
I found versions 5 and 6 really buggy. 7 got better but I am waiting a while before going to 8. It is just too risky to upgrade so soon, given its years of buggy releases. It's poor.
