A very similar point was made in the blog comments, but I'll restate my response here in brief:
Concurrency is hard even when you're just using a pencil and a piece of paper. If you write down a description of a system using any kind of simple notation, it will be difficult for me or anyone else to read that an have a good intuition about the runtime behaviour of that system. This isn't a discussion of statefulness or mutability. It's just about the presence of a very large number of interactions that essentially defy any usual sense of causality.
Concurrency is hard even when you're just using a pencil and a piece of paper. If you write down a description of a system using any kind of simple notation, it will be difficult for me or anyone else to read that an have a good intuition about the runtime behaviour of that system. This isn't a discussion of statefulness or mutability. It's just about the presence of a very large number of interactions that essentially defy any usual sense of causality.