Hextech gunblade costs 3400 gold and its the 2nd most expensive item in the game. Trinity Force is the most expensive at 3703 gold. Will of the Ancients costs 2500. That's 900 gold I can put away sooner for a Zhonya's hourglass or Rabadon's deathcap or a Spirit Visage. Or (God forbid) buy some wards.
Hexttech gunblade gives hybrid damage output with +40 AD, 20% spellvamp (which is unique so cannot stack) and +10% lifesteal and an active use that steals hp outright and slows them. Will of the Ancients trades away all of this for 10% CDR. It also heals you for 15% of the damage dealt to the enemy which (if stacking AP) turns out to be GREATER than the 20% spell vamp given by Hextech Gunblade. And it is calculated BEFORE resistances so it bypasses those enemies who stack MR.
Will of the Ancients is great for mages. It is meant for mages. Mages don't need AD since their power comes from their spells. Anyone who does rely on auto attacks and deal hybrid damage typically have an AP multiplier on their attacks or CDR reduction tagged onto their auto attacks. Examples being Diana, Gragas, Kayle, Kennen, Lulu, Nunu, Karma, etc.
Hextech is really only recommended on Akali since she makes full use of the hyrbid damages and can use some extra CC in her kit. Otherwise it is not worth considering as a "smart buy" option on anyone else. Mordekaiser builds AD now so BotRK is a more reasonable buy. Zac shouldn't be building AP to begin with, he scales better with health and spirit visage is a smarter buy on him. If he does build AP it's typically an item like Twin Shadows for the active or Liandries or Rylias for AP without neglecting health. And WotA is the core item for a Vladimir. It gives him everything he needs: CDR, Spellvamp and AP. The only things it is missing is MR, health and armor and he gets those both from his next two items: Spirit Visage & Zhonya's Hourglass.