Speaking as a mechanical engineer, it would be hard to get a definitive answer. The loads depend on both torque and engine speed. Horsepower is the product of the two. Torque relates to the primary forces of combustion - force on the piston and thru the connecting rod. However rpm brings in imbalance forces, which can be larger at very high rpm. Both forces cause failures from "fatigue" which is applying the load many times, so the claim "all racing engines eventually fail" is probably true. You add in statistical variation in small sample numbers and hard to be definitive. The engines vary in balance and makeup of the block. Cast iron is very sensitive to even a small occlusion in the material, which can be a site where a crack starts. Cast iron parts are probably much less predictable in failure than forged connecting rods, which are a more homogeneous material with more reproducible manufacturing.