I've heard several people complain of "shifting fatigue" during long brevets. Jan Heine (Bicycle Quarterly) has noted it among STI/Ergo users as well, but he's such a retro-grouch that you have to take him with a grain of salt on some of this stuff. I don't use brifters myself so can't comment from personal experience.
I have had arm and hand cramps from braking on long, steep, unpaved downhills, to the point that I've put together a bike with a drum/drag brake for such rides. FWIW, I'm a competition pistol shooter so I do extensive hand and wrist exercises daily. In my experience, hand strength alone isn't enough to guarantee you'll have no problems with controls on the bike.