Drawing the boundary at high throughput, huge fan-out and ultra-low-latency is correct - I'd also add that MQs are often used for pub/sub and signaling.

MQs are heavily optimized for reducing E2E latency between publishers and consumers in a way that DE engines are not, since DE engines usually rely on an ACID compliant database. Under load I've seen an order of magnitude difference in enqueue times (low single-digit milliseconds for the MQ p95 vs 10ms p95 for Postgres commit times). And AMQP has a number of routing features built-in (i.e. different exchange types) that you won't see in DE engines.

Another way to think about it is that message queues usually provide an optional message durability layer alongside signaling and pub/sub. So if you need a very simple queue with retries _and_ you need pub/sub, I'd be eyeing an MQ (or a DE execution engine that supports basic pub/sub, like Hatchet).

I wrote about our perspective on this here: https://hatchet.run/blog/durable-execution

( disclaimer - I'm one of the people behind https://github.com/hatchet-dev/hatchet )