As support main i can assure you, that Support can't carry alone. Sure a good support means a hell of a difference, but will not win the game without at least one good teammate.
Poker supports need the ADC to poke as well. Otherwise their trading will end up badly. Assuming the enemy is not an idiot.
Tank supports need the ADC to ract the engage. Otherwise they just end up 2v1. Not engaging will lose the lane against poker, and protector support. Fast agains poker, and slow against protector.
Protector support needs a teammate they can work with in teamfight. Someone worth protecting. That person is not neccessarily the ADC. Better give everything to a fed Yi, than a worthless Jihn as Lulu.
As for building. They still got less gold. It's pretty much the weakness of the poker supports. If they fail to get some kills/assist early, then the lack of gold makes them worthless until late. I remember non-fed support Lux putting out whole combo on ADC, and dealt like 40% damage. Took like 3 seconds for him to get it back from lifesteal. As tank building QSS isn't much problem since mr. is good anyway. As protector building QSS means delaying other important items. Though their lack of items is not that critical.