1. Head-of-line blocking caused by congestion on the single TCP connection used for message transmission between any two Erlang nodes. This can cause major problems for apps control vs. data plane design, such as Riak and Lasp. Work on Partisan (https://github.com/lasp-lang/partisan) appears to be a substantial improvement.
2. If the single remote distribution TCP connection between two nodes is broken, then the first Erlang process to send a message to the remote node is, hrrm, well, borrowed/co-opted by the BEAM VM to connect the new TCP session. IIRC that process is marked unrunnable by the scheduler until the connection is set up or perhaps there's an error. If that process is really important to the functioning of your app, for example, an important system manager in the control plane of the app, then you have a very difficult-to-diagnose latency Heisenbug to cope with.
-Scott
More generally, it doesn’t work when you have multiple schedulers, which nowadays is true for nearly all Erlang systems.
I’ve heard speculation that it really only helped scheduler efficiency and this didn’t apply all that well on SMP schedulers and has finally been removed since non-SMP builds are no longer supported.
As for Pony, their actor model doesn’t support selectively receiving messages so their queue handling may have some different trades to make there, as well as interaction with primitives like promises (which are used where Erlang would prefer a ref tagged receive clause). Spooneybarger has done work on backpressure in Pony so maybe he or other Wallaroo labs people can elaborate.
I never speak of either being better than the other. Rather, they are different. I'm an expert on backpressure in Pony, not so much in Erlang, so I'll leave it at that.