I love the language. I'm using Vapor+Fluent+Leaf, but I've had a few issues with them, I think they're fine, but if I was making decisions again I'd probably choose something else. I think the problem is that Vapor is trying to be a Django/Rails style, batteries-included framework as opposed to a Flask-style library (if those terms help you at all), but they've not quite nailed it to the Django/Rails level yet. For my project I'd have been better off with less framework, as most of my code is not strictly web code. I've ended up changing the Vapor application architecture a bit and pulling lots out into non-web-specific Swift Modules. If I were choosing again I'd give consideration to Hummingbird.

Library availability has mostly not been a problem for me, there are some really high quality libraries, particularly open source ones from Apple. Quality is high compared to other ecosystems.

I'm targeting Linux/Docker for deployment and while I don't have the project running in production yet, I've been running CI builds and it so far seems fine. It's fairly clear when things are Linux compatible and when they're not. Most things you'd expect to be are compatible. Some third-party libraries are accidentally not Linux compatible, but it hasn't been much of a problem for me.

The new Swift Testing library is really nice, and I'm using that for all my server tests with no real problems. Vapor could have more testing support (Django's is excellent and my bar for comparison), but it all works.

Overall it's just a lovely language to work in, and I'm enjoying server side development with it.