There’s a level of self fulfilling prophecy here: people don’t use Swift off Apple platforms because there isn’t a critical mass of people using Swift off Apple platforms.
But that said it can be frustrating. A lot of the documentation and tutorials you find out there assume you’re on an Apple platform where, e.g. Foundation APIs are available. And the direction of the language, the features being added etc, very clearly serve Apple’s goals.
None, if you stick to using the bits from swift-foundation instead of swift-corelibs-foundation. Confusing, but the new code is much better, and in production on Apple's platforms as well.
But that said it can be frustrating. A lot of the documentation and tutorials you find out there assume you’re on an Apple platform where, e.g. Foundation APIs are available. And the direction of the language, the features being added etc, very clearly serve Apple’s goals.
(Side note: IBM was an early adopter of Swift off-platform but then stepped back: https://forums.swift.org/t/december-12th-2019/31735)