The restrictions in leg room can come due to the misaligned seat anchors of the seats in front and the seat you are sitting in. The seats in row 56 are off-centre from the seats in front, with the seat anchors potentially taking up some of the foot space. The advantage of row 56, depending on which side you choose (and you want the one where there's a gap from the missing seat), is the side space. It varies between airlines as to the side this is on - some have the missing seat on theleft aisle, some on the right.