The author's examples of rough edges are however no better when hosted on vercel. The architecture seems... overly clever, leading to all kinds of issues.

I'm sure commercial incentives would lead issues that affect paying (hosted) customers to have better resolutions than those self-hosting, but that's not enough to explain this level of pain, especially not in issues that would affect paying customers just as much.

I agree, but I suspect the cleverness is part proprietary behaviour, part having a monstrosity that only runs as intended on their own infra