I have no doubt that you could break someone's neck with a swing from a mace and destroy their skull, but the fact the head is held on pretty damn well with a lot of strong and flexible tissue means you're not going to knock it clean off without some velocity of which humans simply aren't capable.
My dad was a wrecker driver when he was growing up. He saw a car after it had hit a tree. Some of the occupants were decapitated from the impact force. It can happen with enough force applied to the body or the head. Remember, this was back in the day when there was no crush zones in cars, so the full impact G Force was applied to the people in the car. People back then were messed up with car accidents, not like today.
However, the decapitations are just one more finishing move, so it does not break immersion like the Bloody Mess perk did.