Cross-platform stuff is mainly non-GUI since the main thrust of the cross-platform scene seems to be server-side Swift, and most of the GUI stuff is targeted to iOS (although with Project Catalyst, that's all implicitly valid macOS code now).
I expect we may see some third-party, cross-platform reimplementations of SwiftUI very soon once the appropriate language features are set in stone and ship with the next version of Swift.
Otherwise, it's fairly healthy — lots of computational libraries, database, web, etc. Plus, Swift pretty much has direct compatibility with C and Python libraries without needing to wrap anything.
C compatibility is one of the original design goals for Swift and is used extensively for the Linux version, for making Swift wrappers around existing libraries, and the Objective-C bridge on macOS. If using Xcode, one would use a bridging header; otherwise, one can use Swift Package Manager to wrap C libraries for use in Swift projects.
Swift recently gained the features needed to call Python code. You can either use the Python module from Swift for Tensorflow or the PythonKit library, both of which allow calling Python code from within Swift.
The features used to allow calling Python from Swift can also be used for other dynamic languages, so there may be the ability to call Ruby and other languages' libraries in future as well.
I expect we may see some third-party, cross-platform reimplementations of SwiftUI very soon once the appropriate language features are set in stone and ship with the next version of Swift.
Otherwise, it's fairly healthy — lots of computational libraries, database, web, etc. Plus, Swift pretty much has direct compatibility with C and Python libraries without needing to wrap anything.