We uses ZeroMQ on production as part of a message layer which provide very tight and predictable latencies sustaining more than 300K QPS across multiple services and across 3 data centers(GCP, AWS and private Datacenter). This is a stellar testament of how good ZeroMQ is as a mature solution.

I think the reason we are successful with our design is because ZeroMQ is just great for what it does. It never crashed or gave problems to us, only issue we saw was because we did not understand how it worked internally.

Had it been written in C, we would have been able to hack it a little bit to our tastes but I am not complaining. I can understand why someone would use C instead of C++ for high performing libraries.

Oh on the subject of C:

This is what we have :- Main service which is written in C does around 8K QPS at peak hours(on a 2 core 4GB Amazon EC2 instance), so we have around 35-40 instances in the cloud. The cost to run this service is low for us, we love it.:)

Biggest issue, people are bad at C and writing correct multithreaded code in C is an Art.