Elm already was the gold standard when it comes to helpful error messages and it's amazing to see that they're still working to make it even better. I hope other languages follow suit.
I use shairport-sync on my iMac to stream Overcast from the iPhone. There is a huge latency and the connection drops out every once in a while. This should have been a built in feature.
Sidekiq was recently ported over to crystal and the speedups and memory usage improvements are amazing. There's also an officially supported heroku-buildpack so it can be deployed easily.
But there's a lot of breaking changes in the language (0.18 was released a few days ago with a few breaking changes). I would not use it for a long term project (yet). But for some personal project or prototypes, for example which you'd quickly set using Ruby/Sinatra, I'd choose Crystal/Kemal.
Crystal: Syntax inspired by Ruby, static types and compiled by LLVM. The standard library is mostly similar to Ruby (I'd say it's better. For example it rack-like functionality and websockets built-in). It's getting a lot of traction in the community.
I'm actually writing Elm at work and we have two separate Elm apps in production.
YMMV but Elm with elm-graphql has been the most productive and confident I've ever been writing frontend code.