First of all, in normal use or even high performance use, a used crankshaft will not “clean up” to standard specifications and will usually have to be ground to .010? undersize. The crankshaft has then remained 98+ percent as strong as new. The problem arises when “spinning” or “hammering” (knocking) a bearing.
The crank will not clean up at .010? and must be ground down to .020? or more. A spun or hammered journal causes a crack in the majority of cases, even though it might not appear to be badly damaged.
In order to detect a crack, the crank must be crack checked or “magnafluxed.” Some crank rebuilders do not crack check their product and depend upon the “law of averages.”
The backyard mechanic/racer was right when he found that crankshafts ground past .010? would have a tendency to break, assuming it was because of the reduction in strength and not aware that it was cracked already when it came from the crank repair shop.