If ADCs are supposed to be the easiest class to kill then why are they allowed to build DD
(Cough, Draven.)
Seriously though. The tradeoff to their high sustained (and even burst, cough, cait draven) is that they will die to one lux combo, one riven combo, a 5-ball syndra ult, even a Brand combo.
Why are they allowed to build Deaths Dance and then become unkillable with 400+ lifesteal autos and unburstability? This is stupid. ADCs should be punished for bad positioning and whatnot. This is entirely negated by building this one item that is basically an ADCs dream.

