I've got a few books specifically about injuring people, specifically one that was written by a doctor as a guide for writers about how to hurt characters and what specific damage and weapons would do. Combining that with the knowledge I have of physics, I'd say that the bar mace would be most effective when:
Striking to the head (Top of the head is best, but really anywhere works. The face is kinda delicate), throat (specifically the windpipe or around the Carotid sinus), Shins (the front of the tibia is quite near the surface of the leg and has little padding from muscle or skin. It also has a big fucking nerve running straight down the front. A blow to the shin is bad news bears even if it doesn't break a bone.) hands, genitals or shoulders, specifically near the neck so you can hit the collar bone.
Blows to the arms work alright, upper legs are too muscled and padded for much, and the rip cage will protect the chest, though you may break a rib or two. The gut might cause internal bleeding and will certainly cause a lot of pain, but isn't very fatal, at least not quickly. The back is too flat for you to get a good, focused enough force to break the spine. Knee caps work, if you can manage to be that precise.
Like many blunt weapons, you're really going to be aiming for the head unless you're trying to incapacitate them first.
Personally I just like it because
1. It solves or at least lessens the COB problem that maces generally have, making it theoretically easier to use.
2. It's rarely heard of and even more rare to see, existing only in a small section of Europe for a short period of time.
3. I dunno why, but it just feels like it would be a real bastard of a weapon. There's an inherent brutality to it, like a baseball bat wrapped in barbed wire (In actuality that is a pretty shit weapon).