I would agree that APIs are more of a kind of UI than a "something else" where the rules are different. They can afford to scale up their complexity more than

I would agree that APIs are more of a kind of UI than a "something else" where the rules are different. They can afford to scale up their complexity more than graphical designs because textual reference lets you bury stuff very deeply, but the user still has to deal with the "conceptual load," which in a big API can be pretty intensive.