The problem with certificates is, that you really have to look very closely to understand what was covered. For example Stanford has a range of certificates, some taking as little as about one day of work, the others a few semesters. There are so many factors involved such as professional, undergraduate, graduate, for-credit, for continuous education credit, and so on. Therefore I became somewhat skeptical of them as they almost always require more explanation than available on a CV (if we are talking brand value here).