Counterexample: I once helped a fellow recover a stack of 8’ by 4’ plywood off the road after he drove off a little too enthusiastically, tailgate down.
If I have bags of groceries in the truck bed, they will fall over and scatter around. If I have a box of nails there, it’ll slide a bit but not spill. It’s also what you carry that affects the stability of the load. Bowling balls, I suspect, would be difficult to keep in the truck with the tailgate down no matter how gently you drove.