Is reving an engine to redline without a load (in neutral) bad? - Engine & fuel engineering - Eng-TipsAs pointed out by others, redline with a 4 stroke at exhaust TDC subjects the rod to ~ max tensile loads, but running no load would likely make the former firing TDC load similar, with a slight discount of at least compression pressure X piston area. The end result would ~ double the rate at which those max loads are accumulated.
I don't think the valve train loads are much different under power, except maybe the force required to crack open the exhaust valve.
A lot, if not most rod breakage (and metallic part failure in general) is the final stage of progressive cracking from fatigue. In the days when lots of passenger cars were still using forged steel rods I used to magnaflux rods at a hot rod engine shop. It was common to find small "indications" in the area brutalized by factory machining to accept the rod bolt head. That was true of abused engines as well as used passenger engines. It was much less common to find indications along the I-beam or around the wrist pin eye.