It seems the internet shares this opinion. My feeling was that `px` was a general solution, but I may have misunderstood the true meaning of em. I'd imagined code such as:
em = 16
title = Text.height (2*em) (toText "Title")
pic = image 400 (4*em) "yogi.jpg"
I can also imagine the definition of em depending on the user's settings or fonts. In any case, we'll definitely open this up for discussion on the lists again if people start running into problems :)