I think it's something to do with the skin on the face being more sensitive than the skin on the rest of your body, and not being made up the same (I think I'm just talking balls now really but seems to make kinda sense).
try it pink. Use it to draw pattern on bf's beard. See how many days it shows for as it tries to grow back.
make sure you use a sensitive one, for bikini areas.
If you really want to try it, you can get special Immac (or Vite or whatever it's called now) for your face - specially formulated for facial use and not as strong.
When I was in the forces, been out 30 years now, we had two chaps who tried it, they came out in sores all over where they had put it, and ended up on a charge, Self inflicted wound.
A long time ago, but with advances in technology, there may be one out there.