200+ popular business app integrations on MailChimp.
While em is relative to the font-size of its direct or nearest parent, rem is only relative to the html (root) font-size.
Jeremy tends to favor em, because of the ability to control an area of a design. As in, scale the type in that specific area relatively. I have tended to like rem because of the ability to scale type across the entire page easily, but I’ve gotten into issues where that wasn’t good enough control so I could see moving back to em for that reason.