Wyverns

I would say the opposite – Dark Wyverns are good for a quick game, or if you have 15-20 of them. They regenerate and their poison is helpful against smaller creatures. But if you have 20+ Paoka, they will be more effective, and their attacks will be stronger than the meager damage from the Wyvern's breath, making it worthwhile to eat corpses.

Added 2 minutes later
Imagine this: an Orc versus a Haven in a rush. You have 15 Paoka, and they have 15 Knights. They attack your Paoka once, and you don't destroy anything, and there are no corpses. The Knights will quickly defeat the "goose." But if you have a Dark Wyvern, it will at least heal itself a bit during its turn, and it will also poison them, dealing 15-75 damage, which is almost like a Knight. It's more useful, and this Wyvern has a higher attack.

Added 2 minutes later

I don't see much use in the poison, although it might be good with a large number of Wyverns. But electricity is more useful. Considering the corpse-eating, which not only revives units but also prevents the enemy from raising the dead, my choice is Paoka.

If you're playing against the undead, then Paoka are definitely a better choice than Wyverns. Poison doesn't work on the undead, and regeneration won't save you. Feeding the Wyvern bones is a good strategy, especially if you only have one stack of Wyverns left, and the enemy has 2-3 stacks of smaller units. Fly around, eat everything, and even throw some retaliation spells.