I also liked them a lot at first, but they lack two common advantages of level 7 units: speed and flight. Although the energy rune partially compensates for this deficiency.
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I once voted for both Black Dragons and Archdevils at the same time. Since then, my opinion has changed somewhat, and now I would compare Archdevils and Archangels. Although Black Dragons are stronger than Archdevils, the latter are more useful from a tactical point of view, as they have significantly more initiative. And Archangels are also useful from this point of view thanks to their resurrection ability.