The universe didn't offer a manual on how to play guitar, so how are you determining that their technique was bad? Given what you say about them, maybe they actually had the perfect technique?
Given a certain set of quantifiable measures that is no doubt true, but then that only pushes the question to how are the measures determined to be objectively relevant? If the aforementioned fast/famous guitar players had started with a different technique there is a chance they wouldn't have become fast/famous. In that case, given the criteria of reaching notable speed/fame, it is possible their "bad" methods were actually best of all.
But also, even where everyone agrees there is a better way, that doesn't equate to an alternative being bad. So the original question still stands: How do we determine "shockingly bad" as opposed to "different"?