Soarin' is fine, very smooth. There is a sensation of moving but other than the initial rising of the seats, and a slight tilt left or right during the ride, it's the movie that gives you the impression of moving, sort of like the sensation you get standing in the China circle theater in Epcot. Close your eyes and you feel nothing.
POTC has a mild splashdown on the water drop, but it's not really jarring. If you totally tense up for that one it's worse than just relaxing and enjoying the ride.
One that does really jerk and bounce around that you wouldn't expect is Dinosaur. My back and neck ache after that one since I have arthritis in my neck, spine, and hips. The front of the car fares a little than the back, but I'd advise you skip it. The safari jeep in AK bounces around a bit, again better sitting in the front, but the road is rutted on purpose to make you feel like you're in the jungle. Expedition Everest has quite a jerk when you go backwards. Toy Story really whips you around fast a few times early in the ride, tough with a neck injury.