I use Swift for cross-platforms applications successfully, i.e. they run on macOS, Linux, and Windows. They are currently command-line only, but I have planned a Windows GUI for one application, I hope The Browser Company will release a nice GUI library for Windows soon. For Linux there e.g. are some wrappers around GTK, see this other forums topic. But I think you should not hope for a good single cross-platform GUI library soon, you certainly will have to do some re-implementation of the GUI for each platform family.
Yes, the Linux and Windows versions of Swift sometimes lag behind the version for Apple platforms (regular expression literals, macros). You might also consider how “secure” the future is for other than the Apple platforms in the long term. From what I heard Apple itself is using Swift on Linux. A while ago some doubts might have been justified for Windows, but I think now the future looks very good even for Windows, even Apple’s Swift team is very positive about Swift on Windows and even help — but you will have to judge for yourself after being part of the community for some time. Programming cross-platforms application will be much nicer once the common standard libraries are finished.
The interesting part about using Swift as a cross-platform language is that I think Swift could be a very nice “successor” to C++, and Swift is better accessible for beginners than Rust, which is often treated as another candidate in this space. And the Swift implementation on Windows is very interesting, because it integrates natively into Windows so to speak, making it a “true native” choice for Windows. And of course you can “naturally” program apps for Apple’s devices.