I've built several production quality, mid-sized apps in Phoenix/Elixir over the last year using Claude Code (CC).
In my experience, it's great, I had better results with this stack than I had with Django when using purely CC.
It's token efficient (because of code generator), has great static analysis, and terse (because functional). Phoenix even generates an Agents.md
The apps themselves are highly scalable and look great.
Checkout https://github.com/oliver-kriska/claude-elixir-phoenix (not me).