In my experience playing against Deamons, I'd say they are not underpowered so much as just difficult to play. I would say the most important key to victory with most Deamons is simply deployment. For some reason, most of the Deamon players I have seen are playing on a reactionary basis, Deep Striking in specific units to deal with specific threats, as opposed to using Icons effectively, bringing in the majority of your force in a coherent formation, and executing a battle plan. Those who do the latter seem to do fine with the army.
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