Whitewater class ratings are somewhat silly IMHO. Yes, boat size and design can help but ultimately it will boil down to the whitewater skills of the boat handler.
There are Class III rapids that have large standing waves and rollers that, if hit incorrectly, would easily flip a WM, WS or Scadden (choose your model). The Ottawa River comes to mind with some huge "friendly" Class III rapids that will flip a 9' raft.
We own WM Kodiaks and Assualt XXs. The Kodiak is shorter and has no rocker when compared to the XX. It is much more prone to flipping. More concerning is that it handles terribly since it doesn't have rocker. I would only take a WM Kodia/Waterstrider on easy Class III. Even then better make sure you take the correct line.
For the Assault XX I would take it on heavier Class III. But again better take the right line. I wouldn't feel comfortable taking it on Class IV.
All these pontoons have the integrated oar locks and small, flexy paddles.
If you want to run serious whitewater I'd get a real whitewater raft (Aire, Sotar, etc), real oars and real oarlocks.