The distinction is really not queues or not but unbounded in theory or not. There are queues everywhere, connection pools, unparsed messages, packets in flight,

The distinction is really not queues or not but unbounded in theory or not. There are queues everywhere, connection pools, unparsed messages, packets in flight, memory for calls sent out and not yet replied or timed out, etc. but one unbounded (in theory; they are never unbounded in practice) queue can make your system go from robust to fragile.