Whenever the topic of balance comes up in Facebook's Kings of War Fanatics group, the general consensus is that Kings of War is actually fairly well balanced. No one really claims it is perfectly balanced, and very few people claim that it is broken.
But what do people mean by this? Everyone probably has slightly different definitions, but for me a system is balanced when the power curve relative to the points costs is fairly consistent. That is, if you plot power against points, you will get the units clustered fairly close to a line. This line might be a straight line, or a curve, or a wiggle, but the units tend to stick fairly close to it.