Is Swift on the server a good idea? I don't own (or plan to own) a Mac. Does Apple support cross-platform tooling? I honestly don't know, but given their history, my default assumption is to be skeptical. Why would you choose this over Go or Rust? (Rust is totally sweet for server dev, and I've spun up a few Rust servers for things.)
In all honesty though, even though I really like Swift, I'd probably choose something else I already know well like ASP.NET Core until the Swift-on-the-server scene matures.
As for cross-platform capabilities, IBM has contributed a lot of effort to running Swift on Linux and in the cloud. They even have a web-based REPL you can check out.