There's this word I picked up at school Haecceity ... sort of means thisness.
So, the 3V recipe and instructions for use might make it the toughest thing, but the one you get might not be, is all. And, in the past, it has been possible, by keeping an eye on reviews and reports of actual world experiences, to track who might be making the errors.
Yeah, I heard about Bark River (I think) having numerous issues with their 3V heat treatment a few years ago.
You’re right though, you could buy the most statistically reliable car on the market and it could shear a valve head within the first month. I suppose you’re just reducing the chances of this stuff happening by following the science and reviewing the test results.