Instead of talking theory, I'll just give my story.
I originally built Sleeperbot.com in 2009, a realtime fantasy football forum in node.js and it was great for the first few thousand users, about ~100-300 concurrent at anytime.
However a lot of things bothered me, like error handling, lack of solid libraries (major bugs in socket.io and zmq), which made me switch to Python for productivity & stability reasons.
The app scaled really well in Python for a bit, I was able to address most bugs in Python pretty easily compared to node.js. However the users grew to 20x, and I experienced scaling issues with many thousands of connected clients all trying to get in with 10 seconds of each other when we send a global Push Notifications. Both Node.js and Python would have this same problem, being as they are single threaded, so you'd have to write your own distributed fault-tolerant system which is not exactly simple.
This ultimately led me to Erlang. The syntax for Erlang was ugly, but I didn't care. I just cared that my product was stable in production and can handle 20,000 concurrent users all connecting to a machine within a 10 second window and passing messages back and forth in real-time. I needed something that could work just as well for 100,000 users which I expect next year. I also cared that it doesn't crash and that it's self-healing.
The thing I learned is this. If you care about productivity, sure, by all means, node.js. If you care about production and stability, go with something proven, like Erlang.
I've settled on Elixir over Erlang because it has better syntax and compiles to bytecode that runs on the Erlang VM. It also has more momentum than Erlang right now. The cons are that you will experience productivity loss when you switch from OO to Functional style of programming in the very beginning. Elixir / Erlang being less used languages are also missing some libraries that Python and Ruby have that make things dead simple, so I have to be a bit of a polyglot in the meantime.
I still use node.js for mostly brunch.io tasks, but that's about it.