Tooling is all right, not as good as the cross platform tooling for something like Java, but still pretty good, especially given how recently Swift has had any non-hobbyist presence on non-Mac platforms.
The core language "just works" pretty pleasantly on other *nixes, I don't know about windows.
You end up missing Apple-specific libraries a lot, though. There are some packages making progress on that problem, but a comprehensive replacement for e.g. audio or graphics surfaces is a ways off, I think, if only because of the fragmentation that exists in those spaces on other platforms. Of course, Swift has that problem only a little worse than any other language that ships without an OS.
It's really coming along now with Swift 4, but it is still (of course) behind the Apple platforms.
The "core language" pretty much works great on Ubuttnu, but there are still minor annoyances getting it installed, or building it, on Linux. Other Linux targets, also doable, but more annoying. Windows, I haven't really heard of people doing for real. Not sure that's even on anybody's radar.
What's really awesome is how far Swift has come with the core lib Foundation, which is a from-scratch clone of Apple's Objective-C and closed-source macOS/iOS Foundation library[1], written in Swift, in the open. It is not done, but huge swaths of it are now done[2], and that makes coding in Swift on Linux a lot more pleasant, since we don't typically want to waste time rolling our own regex support, Unicode string manipulations, basic geometry routines, HTTP networking, date formatting, etc. But Foundation is a huge, almost 30-year-old library that has evolved continuously and has shipped with every OS X/macOS/iOS release. An open-source Swift re-implementation of that sounded like "yeah right" bullshit/fantasy when they announced it but now it looks very real just a couple years later.
Tooling is different issue, though. Swift is a modern language with excellent support for auto-completion and in-editor hinting and help. People complain about it all the time, but Xcode is so much better than any IDE on Linux (or any other platform) for coding in Swift that I do most of my Swift-on-Linux work using a Mac, with Ubuttnu running in VMWare Fusion. If a basic editor will do, though, you have a lot of choices. For builds and package management, Swift Package Manager is now built into Swift, and works great. (It also has an option to automatically generate Xcode project files from Swift packages, which is hugely useful; my practice, and I think the prevailing or at least emerging convention, is to keep the package and source in git but treat the Xcode project as a throwaway item, not in version control, that can be generated whenever needed. So you don't need Xcode, but its easy to use for editing convenience when you want, assuming you have a Mac. But not all devs need to have Macs.)
I think you still have to like Swift a lot to choose it for your Linux project, but it is doable. I'd be surprised if Swift adoption on Linux didn't quintuple or sextuple by the end of 2018.
The core language "just works" pretty pleasantly on other *nixes, I don't know about windows.
You end up missing Apple-specific libraries a lot, though. There are some packages making progress on that problem, but a comprehensive replacement for e.g. audio or graphics surfaces is a ways off, I think, if only because of the fragmentation that exists in those spaces on other platforms. Of course, Swift has that problem only a little worse than any other language that ships without an OS.