Our group has been slowly playing through the Shadow Rune campaign as an alternate game when our full group of RPGers doesn't show up. We just finished the first Act 2 scenario and have come to the conclusion that the game is far too stressful. It keeps both the Heroes and the Overlord on the edge of our seats for the entire session and it doesn't feel like there is a clear winner till the very end. It seems impossible to predict who will win throughout the entire game!
I just wanted to chime in and say that for our group the game has not felt unbalanced in any way, and the scenarios play out in a tense, suspenseful and very entertaining manner. Well done FFG!