They are mostly late game and are about sustained damage. They suck if game end too early or if game is too bursty. Asking to buff ADC is the same as asking to buff AP because you can't last pick AP in a full AP team, that would just make AP too strong in a normal team comp. ADC player need to learn to play champion select properly, it doesn't make sense if the smalled archetype are good in every team comp. If they are good in a short and bursty game, which is supposed to be their weakness, they become OP under favorable team comp.
PS : I am a ADC main (Vayne, Ashe, Ezreal, Kaisa)