The one avantage ems have over px is that you can set a base font size in em, and then adjust all fonts on mobile by adjusting that base font size.
However, we’re not working with a ton of different font sizes throughout the theme – there’s only a handful (body, site title, tagline, navigation, widget titles and headings).
Therefore, I don’t see a huge advantage to using ems over px. It’s pretty easy to adjust specific fonts on mobile using some CSS, and you can choose to only adjust a couple over adjusting all of them.
Let me know if you need more info 🙂