Usage of rem for vertical rhythm

[krulik]

I've noticed the suggestion of the use of rem in the code for vertical rhythm, with SASS calculations based on font-size. I'm wondering whether usage of em for heading margins wouldn't be more useful and simple - effectively the same calculation would happen automatically. In this case the dynamic nature of em size is actually a feature - larger heading will get more space, etc.
So I suggest to revise the rem suggestion, or at least to provide some useful information about the em alternative in this context.
Obviously you know a lot about the subject, so I also wonder maybe there's a thing for me to learn here.
This article (which is linked in your page) provides examples for this use case:
https://zellwk.com/blog/rem-vs-em/