The stability of a large saddlebag depends on how well it is mounted. The Carraduce bags are mounted transversely and if the straps are buckled on the inside of the bag (rather than on the outside as shown in many photos) so that the bag is tight against the saddle and then the third strap pulls the bag tight against the seat post then sway is tiny. The longitudinal mounting if the bikepacking bags might amplify any sway and I have seen videos where they bounce about a lot. But I imagine correct mounting would minimize that and they they also work well when used correctly.
are the straps very durable? I'd hazard a guess that the bag is more stable when mounted straight to the saddle loops and seat post, which has 3 connecting points, as opposed to having the support rack, which really only has one (two if you count each rail), and nothing to keep the thing from bouncing up and down on rough terrain.