Yea, but high % moves are also not subjective. Sure, you might break the knee but how easy is it to hit that?
Knee stomps are high percentage and easy to hit. Did you not see the Jon Jones video? He did it all the time.
Line a bunch of your friends up according to height, and interestingly you'll find despite the range in heights, their knees will be roughly at the same height, excluding obvious extremes.
It's not hard to train to hit the knee with precision, especially on weighted lead-leg fighters with no awareness of leg kicks, like boxers who don't cross train.
Now, I didn't say it will break the knee, though possible, but hyperextension can easily cause lasting injury, and absolutely take someone out of a fight or at least hinder their mobility and therefore effectiveness against you.