I've struggled with maintaining ad-hoc rules for REMs and PX. In my experience, designers very rarely specify units on metrics and assume PX all the way.

How do you feel about the coupling of "text-sized icons" to the size of text?

Does an icon next to a label make the icon's size font-related in your view?

My go-to heuristic lately is: "is this layout-related?" -> it gets the PX. "is this text-related?" gets REMs. Margins and padding are denoted "layout" -> PX.