I'm not convinced for guitar. Some of the fastest and most famous guitarists had shockingly bad technique.
As long as you're not injuring yourself, practice and determination pretty much overcomes everything.
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"?
Piano is also a lot harder to have very bad technique than other instruments since it is mostly a discrete, one-to-one mapping. If you want a chord with some particular notes, a lot of the time there is only a single fingering that will do. If you push the key hard enough, you will get the correct note, and it won't be wrong if you push harder.
By contrast, in guitar, if you push too hard, your note will be off even though you fretted in the correct place. Or, for example, everybody in guitar teaches barre chords up near the nut, when that's extremely difficult and likely to injure a beginner who has neither the strength nor control to get that right, instead of teaching barre chords near the body on fewer strings. etc.