Have had chronic but mild rotator cuff discomfort for about 10 years due to egotistical bench pressing. For more than 30 years I have had at least one ruddered boat and one rudderless. I also have a GP, 2 low angle euro paddles, and a wing. Results of my experiments to determine what is easiest on the shoulder for my usual 2 to 3 hr ocean paddle at slightly less than 4 kts average are as follows:
1) Most beneficial to the shoulder is having the rudder. Over 2 to 3 hrs even occasional steering strokes take a toll, no matter which paddle, and the shoulder aches.
2) Second most beneficial is technique, especially arms with a non-varying bend during power application, and very especially at the catch. It is at the catch that my shoulder is most vulnerable.
3) The GP is, by a very slight margin, easier on my shoulder than the low angle Euros, primarily because the catch is more gradual. If I set a short wing length, 208 cm, my wing stroke is better and, to my surprise, causes no extra strain on the shoulder compared to the GP.