What's wrong with this battle?

[Saber Cherry]

They aren't. Arch devils are certainly better given that they can cast phoenix pyre.

I think they're better, though it depends on the map and nation. Self-casting Quickness, the ability to go underwater, higher base protection, bonus protection in cold provinces, and no weakness to fire (versus 50% weakness to cold, for Arch Devils) make them extremely potent. Not so great for Abysia, though. Also, some people might prefer Fire Shield, but Breath of Winter can be used similarly, against non-undeads.

Neither of them are really as good as Tartarians, though, at 10D+20N each; the differences in HP, encumbrance, size (for trample), and magic skills are pretty major. Immortals such as Wraith Lords and Vampire Lords, and non-standard choices like Vastnessess can all be more effective in many situations, too.