Called shot uses Intellect, Agility and Presence as stats so that might be a reason why it would work less on rages seeing as most of them don't have a massive amount of points in the presence and intellect fields. Than again I never really run rages so I can't tell the difference
I want to tell you called shot and devastating power work great together, all of my glads have these 2 skills...however, i have no glads that are a threat to the throne (this week). I've been experimenting with this build from my beginning,........................................ .........Cain will probably have better luck with them