How sharp do broadheads need to be?

Simply put, the sharper the better.

Blood vessels and especially arteries are thick walled and rubbery. Combine that with the fact that they are in muscle which acts semi-fluid in nature, and they have a tendency to move.

Just as when a nurse is trying to draw blood from your arm using a needle with an extremely sharp point and the blood vessel keeps rolling off to the side preventing the needle from penetrating; the same thing will happen with broadheads if they are not sharp enough.

If you pay attention when performing autopsies on animals you can sometimes see this especially towards the end of a wound channel if the head was losing its edge or was damaged at some point during penetration.