another thing to be kept in mind. sometimes maps are (maybe unintentionally) made in a way that attackers actually are in advantage. the defender have to really good to keep the attackers at bay or even stalemate. whether team a or b is better in the end eventually only show after a rematch on the same map.
L4D2 maybe a bit far fetched but especially the first maps in the dead center campaign have to be won by the survivors. if they don’t the infected are dominating. yet a huge amount of rage-quits in those cases steal the chance to have a real comparison…