You would be astounded the flex and "shake" of the flex joint you can achieve on a 5" ratchet when that force is all that's separating you from your prize. In tight areas the lash of the mechanism, slop of the fastener/socket interface, and this flex can make such a ratchet useless as they begin to eat up the available swing arc.
As Buck said, with high amounts of force regardless of handle length, a non locking flex can be stressful to use. Grab a small 1/4 flex ratchet, and place the head in the vise. The whole head, so the drive mechanism isn't at play. Push/pull on the handle until your arm shakes from maintaining the force, then slightly flex the handle. It will try to suddenly pivot and drag your arm with it. Now with an 18" flex head I have resting on my shoulders as I attempt to stand up and break the bolt free? Yes the sudden changes due to a non locking flex will be much more violent. But even with a flex head 1/4 ratchet these forces can cause rounding due to poor socket engagement, completely slipping off the work, and smashed knuckles.