In my experience, though, a lot of "REST in the real world" failed at its lofty original goals, precisely because its original goals required too much cognitive load.
The reason REST largely succeeded (or, rather, what I like to refer to as "REST-lite") is because people who wanted to build stuff quickly on the web realized "Hey, I don't need all this protocol complexity (see: SOAP), I can just make simple, human-readable API calls over the same HTTP layer my browser uses anyway".
There is other stuff in "official REST" that I think has some value, like the noun/verb structure of API routes, but shoehorning API-level error codes into HTTP status codes has been a disaster IMO. Every time I've seen this done I've seen the same issues come up again and again and new developers constantly have to rediscover solutions and problem spots. Does "404" mean the API endpoint doesn't exist, or that particular resource doesn't exist? How do I map my very specific API error to rather generic HTTP status codes? Does a status code error mean a problem with the networking or the application?