Sharingan do not have resistance to genjutsu, it simply sees through illusions like byakugan through it's penetrative power. CC is not necessary for breaking genjutsu, more power is the factor for breaking a genjutsu by stopping the chakra flow. More power = stopping chakra flow more likely = genjutsu being broken more likely. Intelligence is indeed a factor to see through illusions, but specifically, analytical genjutsu intelligemce as Sasuke praised sakura in part one. Stopping chakra flow is not entirely under the will of the victim, jiraiya said "Try to stop your chakra flow as much as you can by applying more power...", key word = Try. It depends on the power of the illusion user and his/her skill in using it or his/her ocular prowess.
OT:- the parellels never end.