What’s the missing feature? Native M:N green threads? Seems to me that Python’s async is a fair bit more mature than Ruby, so the comparison here is not obvious to me.

The conclusion I have come to is that the way you interact with the concurrency model is just as important as what the model is underneath. Erlang (and Elixir) have got both levels right - M:N scheduling underneath, and the actor model (with excellent library support) on top to best take advantage of it.