Alternate development environments to Xcode?
There is Jetbrain's AppCode but I think you still need Xcode for the GUI (i.e. Xib files). You can move projects from Xcode to AppCode and vice versa so if in a team it will not matter what IDE each member uses.
You can produce macOS apps without using Apple's GUI builder so might not be an issue in all cases. But there is always something I have to tweek in Xcode rather than use AppCode.
For more general comments see this SO answer but note that this predates AppCode which is why it is not mentioned there and so at that time Xcode was the only useful IDE.