Has anyone ever encountered a consistent stress failure with threaded ball valve bodies cracking through the stamp on the hex? If so, what brand(s) and what was determined to be the cause of the failure?
That sounds like a failure with a high correlation to being installed by a gorilla with a pipe wrench.
Those are probably also the low-end WOG (Water-Oil-Gas) valves. ANSI (Now ASME) valves have to meet more stringent material properties and thivkness specifications. See ASME B16.34 and B16.10 for code requirements.