I've only noticed Shopify behind one of the online vendors I use and it has the worst recurring order management UX I have ever seen. Is it that this vendor has set it up badly or is Shopify yet another case of worst in class market dominance?

Recurring order functionality is typically delegated to a third party app in Shopify land; the two biggest incumbents in this space have pretty ordinary UX so it’s likely you were looking at one of those.

More recent entrants in the market do a much better job on this front.