Biggest flaw?
Just rewatched for the first time in ages. One thing I don't think I ever noticed was the soldiers that followed Duncan after the attack on Marian, and that being how they found the village.
They sounded the warning just moments after Duncan arrived. How was the Sheriff able to mobilize a squadron of archers, multiple catapults, and a horde of Celts to follow so closely behind a blind man and not be heard?
4th wall break inside a 4th wall break... that's like 16 walls!