The most difficult alternative

Well, 99.9% of that is wrong. Vampires are faster, and they are only less effective than Poisonous units when dealing with a large number of undead or in battles against golden golems. Otherwise, vampires are much more reliable because you don't need a large number of them, and battles can be fought far from the heroes. The strength of Poisonous units in close combat is extremely low, so sometimes you can sacrifice 1 day for the sake of vampires.