That is clearly one disadvantage of a row frame, they do make frames where the seat is stationary and the oar frame slides to avoid that issue. The boat I built is 19 feet long and you can clearly see the bow move up and down when rowing hard, watch the competition boats and you will see the same thing. Other negatives is they add a lot of weight and bulk to the boat and make them a lot more difficult to handle out of the water. Still all things considered a sliding seat row frame is a lot faster than a fixed seat and oars if speed is your goal.