The only reason conqueror is even decent atm is it gives unreduced healing, that furthermore is unreduced vamp off of aoe skills and stuff. The equivalent spell vamp for aoe skills (assuming enemy has 0 resists) would be 24%. If the enemy has any mr or armor or % damage reduction that goes up drastically.
Even that, at 8%, is too meh to make conqueror strong on its own. It needs the true damage and stat bonuses as it stands to even be competitive for most champions. A handful of champs, however, are good abuser of the rune.