A well equipped Ice Devil has no weaknesses, though there are different ways in which people like to equip them.

With their initial paths in Water (= Quickness and Breath of Winter) they don't have to fill a slot with a quickness item (only Jade Armor or Boots of Quickness) with their double attacks, naturally high hp and good defense and MR they are a better overall chassis for SC's than any other summon (most lack slots).

There is no item that increases initial hp (which is very important) so that is a very important factor.

The AD's would be on par if they had more HP, simply because they are the opposite element they would have their place (especially since they have intrinsic flying and later in the game Phoenix Pyre).

The Heliophagi are just shafted each way.