If you plan to fish in the wind and heavier waves, you need a much greater margin of power than if you plan to avoid such confrontation. Fishing high waves requires you use a sufficiently long shaft to give you the best chance of avoiding loss of power. This can occur when the motor cavitates between swells. One major manufacturer, Minn Kota, recommends shaft lengths of 42 inches if the deck of your boat is from 16 to 22 inches from the water line. As per the point made about rough water, you can simply add five inches to the water line measurement.
Brackets are now available that easily handle heavier thrust motors without any problem.
However, if you want to get the whole story on a shock absorber or breakaway bracket, ask how it does in stumps. To get an answer as to the durability of the various products, go to a repair station and see which ones are having shafts replaced or straightened. If you are going to fish stumps you are not always going to hit a stump head on. More often it will be a side blow and a bracket that lets the motor collapse under the boat will reach a point that the shock absorber effect stops. The Minn Kota folks have upped the ante in design to meet these problems by offering a 360-degree breakaway bracket that reverts to the original position automatically.