For "in production" I didn't mean lines of code, just my personal feelings. I think the SML feature set is more suitable for programs/services with quality/security requirements (like Internet servers). On the other hand, I still like Lisps for prototyping because of the dynamic typing and homoiconicity. Pick the best tool for the job ...
I like Poly/ML. It runs on my Mac M1, supports threads, FFI & compiles to native code. As well as the libraries mentioned on Poly/ML's Web site I find useful stuff in smackage and https://github.com/MLTon/mltonlib (e.g. use-lib for building, contract.sig/sml for design-by-contract).
Unfortunately, this is only for play/personal research now, as >>9 mentioned I get paid for C & Python :-(. But I'm wondering about a side project ...