Well, let us make sure we are talking about the same thing right.
-i agree that 100% resist anything, a side effect of vampirism?, is somewhat over the board.
-i do not agree with the original post and title which says 80% magic absorb is overpowered.

I would go even that far and say its a waste of a stone and a perk. Absorb only gives a chance to completely negate damage. As such, if not 100%, there will be time where the player is hit for full damage multiple times in a row = dead. Magic resistance actually reduces damage to a manageable point. Healing etc. constant effect is always better than a chance. With full magres there is no need for absorb. But with absorb, there is still an absolute need for magres, because damage will go through sometimes.

100hp player, incoming 100damage spell:
magres 85% player is hit for: 15, 15, 15, 15, 15, 15, heals.
absorb 80% player is hit for: 0, 0, 0, 0, 0, 100. DEAD

100% is another matter, but i personally dont think its a big deal and more overpowered than any other thing in skyrim, in played right.

At the time of writing the title, I didn't realise that my atronach effect was necro vamp specific, giving me 100% magic absorption. If you would have just read the comments, you wouldn't have needed to write that.

Normally you get 80%, which IMO is still preferable than 80% magic resist for a mage. You can around 40% magic resist quite easily, then rely mostly on the atronach effect to recover magicka. Also remember that the atronach effect absorbs anything that isn't direct melee damage. Chaurus poison, weapon enchantments, shouts etc.