Piston weight is the biggest conern. Properly prepped stock rods with a light weight piston will last in a lot of cases. As for rod failures, they happen but not from breaking. Usually when the big end has spun a bearing it's either lack of oil or the rod egg shaped under a heavy load, the rod bearing then bites the crank and it spins in the rod. This would be my concern with stock rods in anything other than a healthy street/strip car.