Ems are more versatile. When increasing the size of all fonts at a particular breakpoint for instance. Instead of having to adjust each font-size property individually you can simply change the font-size of the root element.
Absolutely, you can hack it on a very individual basis, but this introduces some major problems. For instance, browsers and operating systems all render fonts differently, and as the web is fluid by nature, simply having a sentence break to the next line can be an issue. Also, it can be incredibly different based on the typefaces you are using.
G'day, I'm the guy that made it. I agree with your comment completely, I had one pretty big constraint though; one element allowed. I'd like to think that is why it is getting upvoted.
The thing to keep in mind is that the use of pseudo-elements keeps the style separate from the content. In terms of practicality, there isn't much; I did this as an experiment in semantics and the latest CSS properties and techniques.
For me, the best way to switch off, and the most rewarding, is to go cycling. Not only is it great for my health, but it is impossible to stay connected.