The most difficult part is managing the delivered / processed state and ordered delivery. Consistent ordering of receipt into a distributed buffer is a great challenge. Most stacks do that pretty well. But deciding when a message has been processed and when you can safely decide not to deliver it again it is especially challenging in a distributed environment.

That is sort of danced around a bit in this article where the author is talking about dropped messages, etc. It is tempting to say "use a stream server" but ultimately stream servers make head-of-line accounting the consumer's responsibility. That's usually solved with some kind of (not distributed) lock.