For example, no debian build of Swift. And when I ported the ubuntu version to .deb, I ran into odd bugs, like when you kill it, it lives on! Apparently a known issue.
Basic _big_ bugs that shows Apple's half assed commitment to open-source on any other platform than their own. I can't blame them, but Swift on linux, beware.
Swift is a wonderful language, sadly I'm not seeing the commitment from Apple to make it ubiquitous. Also to note, Apple has basically killed macOS as a server, rendering Swift more towards a system/UI target language; which is a shame, because again, it's a wonderful, safe, language.