We started using Iron, but later switched to just using hyper because that seemed easier and like a smaller API surface to target. I'm not really working on a web application so I need zero of the routing/web features of these frameworks.
I do need a lot of pretty specific other features though. Unix domain sockets are currently a must (luckily that works with hyperlocal).
The code runs on a pretty resource constrained system so I like the low level of control that hyper gives me of the request/response chunking behaviour (I can not afford to buffer large requests in memory and what exactly I do with the body payload differs between API calls).
One thing that would be lovely is proper TLS/HTTPS support. I need support for x509 client certificates and I need to get access to the full client certificate data, ideally including the full chain that signed it from the request handler. I have to admit that I currently run a hacked together nginx in front of my app and put this stuff into HTTP headers (hence the unix domain sockets) because I could not get it to work with rust ecosystem libraries.
EDIT: The TLS stuff is also true for the HTTP client case. Currently using the libcurl rust binding because it's the only thing that implements all the features I need (--cainfo, --cert, --key, --resolv). Also it needs to run on OpenSSL or something else that supports x509v3 extended attributes (subtree constraints).