The main thing with building
is that, until you have the full item, you only get some stats.
already provides you with a shield against burst damage once you've built one of its components.
Since
also builds out of
, you can even build towards it when you're up against AD in your lane by simply going Serrated Dirk first, and then go towards Maw of Malmortius once magic damage becomes something to consider.
And to top it all off, Zhonya requires precise activation to do anything for you; if you get bursted down 100-0 by Talon before you can activate Zhonya, it's a useless item. Maw of Malmortius and Hexdrinker require no thought from its wielders, and activate passively.
I do still believe AP champions are in an OK spot. However,
provides more magic resistance than
provides armour, and then Maw doesn't inhibit your ability to dodge and deal damage while it passively protects your life, gives lifesteal, spellvamp and attack speed.
It just gives a tremendous lot, such that it's worth building even if the enemy team hardly has magic damage... because once Master Yi's devourer procs your Maw's shield, you instantly get bonus lifesteal, spellvamp and attack speed. You already get 55 attack damage and 10 armour pen anyway.