Crashes caused by turbulence itself are pretty rare. Most were in the 1950s and 1960s. Better construction and better understanding of flying through turbulence has helped.

A signifcant hazard, though, is loose items/people in the cabin. People have broken their necks hitting their heads on the overhead bins. If the plane experiences negative Gs, it's like being held upside down and dropped on your head.

That's why they tell you to keep your seatbelt on at all times while seated.