The downside is that there is low recognition by recruiters, HR, etc. I bought a Coursera certificate for one of them, and posted it to my linkedin account. Had no effect whatsoever. I cannot find it back anymore so it looks like money sent down the toilets. You have to consider it as a hobby.
Also in the corporate world, companies serve low quality, masochistic MOOCs to their employees, which overall gives a distorted representation to everybody and takes the fun out.
I am currently reading online and physical books on a subject I want to know deeper, and that leave no brain juice for MOOCs right now.