The Kubota's during demo's have handled our hilly country side better then the competition has. They are one of if not the heaviest out there!
The weight is in the engine and drives and they are low in the chassis so they have an advantage over the other makes where they are mounted higher in the frame.
Most zero turns only have caster type wheels on the front, so if the rear looses traction than the fronts just follow down the hill!
The commercial tank from Cadet do have some models with front steering wheels and would probably add some security on side hills.