Perception is so individual....I don't think Forbidden Journey jerks me around at all.
I think a lot of people end up suffering from motion sickness and disorientation on FJ, and that may lead to folks feeling like they were tossed around.
I love the Gringott's ride, but have had mixed experiences with the sudden stops. Sometimes they feel pretty smooth, and other times a bit harsh and abrupt.
I'm almost 5'9" and I think only my toes touch the floor in the back rows.
In my opinion, Hagrids is smoother than Gringotts. You can see the ride track easily, and I think that helps our brain to understand what is coming and how to react to the movement. I think the lap bars are similar, but you're more 'enclosed' in the sidecar.