Regarding iOS and macOS kits, all of the documentation is in Objective-C. The libraries' Obj-C is imported and usable with Swift-only code, but the Swift documentation lazily links you to examples in the Obj-C docs. You have to perform trial-and-error by manually converting the Obj-C examples into the equivalent Swift (which often entails renamed classes, methods, types, etc.).

If your projects typically do a lot of string parsing, get ready for a world of hell. Swift 4 didn't improve upon the very basic ability to extract a substring from the middle of a string. It takes 3-4 lines of code, with throwaway explicitly defined variables, to do the equivalent of a str.substr(start, end|length). It boggles the mind that you can't do a str.utf8.substr(3, 5) in Swift.