We use server side Swift extensively since about 2016 for decent production load and it's easily one of the worst decisions I've ever made.
- C/C++ interop is great but if we wanted to use C/C++ libraries why use Swift at all? It's annoying to interop with them even if there is no FFI and still requires a lot of glue code for memory management etc…
- The stdlib (Foundation) is not identical on all platforms even today. This has been a major thorn as releases constantly have discrepancies and subtle bugs that are hard to diagnose and track down. Even Swift 6.1 broke non UTF-8 string encodings by just returning "nil" on Linux and took until Swift 6.2 to be fixed (nearly a year).
- The compile times are awful, with a large Swift codebase it takes us ~10-20 minutes to compile our backend Docker container and thus deployments to dev take that long and it's only going to keep getting longer as Apple seemingly has no interest in making the Swift toolchain much faster and Swift has a fatal flaw in it's design around bi-directional type inference that ensure it can never be compiled fast.
- Talent is impossible to find. Yes lots of people know Swift for iOS apps but nobody knows Swift for server code and a backend dev is a very different skillset than an app dev.
We chose it because it allowed us to share some domain code between our flagship iOS product and the server with a custom built sync engine but as our platform has grown it's just gotten harder and harder to justify keeping Swift on the server which is why we're actively migrating off it.