My memory of them is rather horrible.
Memory leaks, segmentation faults(!) and obscure deadlocks eventually made me scratch EventMachine from the list of things I'd ever want to touch again.
And Faye, well, I don't think I even got it fully running at the time. I still re-call my astonishment when I realized that neither of their two different server-impls at the time would pass even most basic tests (e.g. browser connect/disconnect, redis start/stop) without crashing, locking up or losing messages.
Admittedly it's been a while... Is anyone running Faye in production and can comment?
Edit: Thanks for the feedback. I guess it's time to give Faye a second chance.
Lost messages aren't a concern because it's assumed the protocol won't handle lost or dropped messages, so Firehose messages have a sequence that counts up per channel that the client keeps track of.
And in fact, Action Cable uses Celluloid threadpool to run all the application code in threads. So the model is a combination of EventMachine + Threads.
There's absolutely no reason it can't use Threads + Celluloid all the way. But building on top of Faye was the simplest way to get things up and running.