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.