https://doc.rust-lang.org/complement-lang-faq.html#is-anyone...
Names you might have heard of: Dropbox, Skylight.io, Sandstorm.io, Tessel, Terminal.com, Chef, among others.
Rumprun currently runs on x86 and x86-64, support for RISC-V and ARM is in the making. The architecture specific code is quite small, you just need to make sure that NetBSD (and Rust) support your architecture as well.
It'll be interesting to see what space unikernels take up in the coming years.
I think there is still a lot of fruitful territory to be explored in this space as traditional multi-user, multi-tenant assumptions can be thrown out the window. I think a big, unexplored advance that unikernels might be able to offer is the ability for the kernel-part to peek into the guts of what its programs are actually doing (at compile or run time) and make broader assumptions about scheduling.
Multi-tenant assumptions probably aren't going away, since mainstream systems have by and large not been able to handle them.
Anyway, you are right, not everything is supported. It would be cool to know if a valid program will work on Rust+Rumprun without having to run it, but I can't imagine a simple solution to that problem. I guess if the problem of knowing if something will work without running it were solved, programs wouldn't have bugs ;)