Punching people in the face is generally a bad idea. Not only is it likely to get you in trouble, but the face is one big crumple zone. Jaw, teeth, cheekbones, those are all hard and pointy. Your handbones are fragile. If you manage to put power behind the hit, they are going to have a very visible injury, which can put the observers on their side.
I think this description missed the biggest reason that head-blows are bad. Concussions are long-term cumulative. This includes shakes that are FAR gentler than “comatose” levels of head trauma. When someone gets punched in the head, the world becomes slightly more depressed, slightly more demented, and slightly dumber. Never willingly get punched in the head.
I suspect this heuristic (if adopted) might be easy to Goodhart, or cheaply imitate. I like the intention behind it though.