zmq is used in a many large-scale production systems. It's definitely safe to use and has been stable for a long time. nng is relatively new. Personally I have

zmq is used in a many large-scale production systems. It's definitely safe to use and has been stable for a long time. nng is relatively new. Personally I have not had any issues with it. I'd look through the open Github issues and see if you can find any dealbreakers. The deciding factor may be language/library support. I would also classify it as "safe to use in production" though.