Silly question: isn't the way you're supposed to handle the observer pattern thing to be that you can follow processing chains using correlation IDs? Ie. the initial event of a dataflow gets tagged with a unique ID and then there's some database magic that lets you track that ID through the system? Like, I can see where that would go wrong once you merge multiple events together, but that seems a different failure point than described here.

And while we're at it, the whole point of eventsourcing is that they're not supposed to model "internal" process state? They're supposed to be the ground source of truth; whatever the process has to do to mangle that into a model it can work with should be no skin off the events' back.

The rest seems very, very correct to my experience. Especially not knowing the pain points until you're past the toy level.