Rainer
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I think this opinion about demons is somewhat exaggerated. Maybe they aren't the strongest, but people just don't like playing as them, so they rank them last. They do have a few noteworthy units, and their skills are decent... Level 7 is weak, of course, and that might be one of the main reasons for this attitude towards them, but they can summon Cave Lords, which are quite strong for their level.
And if you upgrade their logistics, luck, and defense, they will be quite good.