They're just really rugged and able to handle just about any amount of horsepower. Additionally, with the removable "pumpkin," it's possible to have more than one "pumpkin" with different ratios sitting on the bench. To change ratios, one pulls the wheels, unbolts the backing plates, slides out the axles and unbolts the pumpkin and replaces it.
(I was typing while Fourdy was posting!)