Fervor of Battle deals more damage than you would heal for with Warlords at 20% HP

At almost all points of the game, 4 stacks of fervor will deal more damage than warlords would heal you for at 20% or less HP on most champions. Unless you build full AD and have 50%+ crit with an , then it takes 7 - 8 stacks of FoB that late in the game.

On top of that, fervor scales with spells in that they generate 2 stacks, while warlord's has no interaction with spells whatsoever.

Both masteries are meant for auto attackers, but fervor takes very little to deal more damage than you would heal for instead with warlords during the vast majority of the game. Late game, it's still stronger it just requires you to have 8 stacks, which is far easier to acquire and maintain than to survive at 20% or less HP.

tl;dr warlord's bloodlust sucks except on Olaf. Needs better scaling to be equal in power to FoB