There's no certifications with these. You can't get an accounting, law, or medical degree, bachelor/associate certificate, or anything you'd need professional accreditation for.
Maybe the completion rate is similar for people who buy books. I'd end up getting a book thinking it'd be an investment as it would be good as learning a subject. The next day I didn't really want to spend 12 straight hours learning Visual Basic.
Also these aren't like auditing a class and listening in on a real lecture either. Just from the list for UT Austin (a very good school): https://www.edx.org/school/utaustinx
There are university lecturers people listen to all the time though, through the Great Courses / Teaching Company. Those sort of fit the leisure-learner path and don't require logging into some system for it.