What's so great about...

[Boron]

I forgot to mention that IDs are Demons . So you can combat them with Flamebeaus or Herald lances .
Sure Demon is better then Undead because they are not affected by Spells but those 2 Weapons are a big weakness for them .

If you have blood you can make 2 Devils with herald lance , ring of warrior + burning pearl for 1 fully equipped ID .
The Devil has also 18 att and 16 Defense , that's roughly equal to the ID .

They should kill the ID normally easily .

The AD has Fly , 100% Fire resistence and Fire magic .
So he can do Fire shield + Phönix pyre , doesn't need fire resistence and fly and has so a slight slot advantage compared to the ID . Also the AD has thnx to his Firemagic better Attack .
So in most Situations he is more useful then the ID imo .

[Oversway]

Don't those devils have like half the hp, 0 natural protection (vs 15 for ID), and ~5 less str? You are right they have better attack and also they have better precision by 2. Defense is about the same. 2 enc vs 1 for ID but not sure that matters.

Ice Devils arn't unbeatable but they aren't pushovers either.