Many of them are fairly faithful representations of the lecture components of courses from "elite" universities. There are certainly many MOOC courses I would struggle with. But, discussion boards are mostly a tire fire (in part, because of the M part as you suggest), problem sets/assignments are limited even if they're not behind a paywall, there are no recitation sessions, etc. So even if someone is motivated, which can be hard in the absence of meaningful certifications, they're just more limited than taking an actual university course.