It is impossible to put a number when it comes to provability.
It relies mostly on patience and consistent practice over a long time.
Injuries happen from ether ignoring what your body is telling you and over training, or rushing things and doing more then you can handle way before you are ready. Form is also important, so injuries from bad form such as wrist sprains are a possibility. Lastly there is injury from not giving the body what it needs to heal properly (rest and nutrition).
In a way it is very similar to wight training. If you load up too much weight to fast then you can hurt yourself. If you have bad lifting form you can hurt yourself. And if you don't do recovery between sessions you can hurt yourself. But, taking your time and doing things right the risk is actually pretty low once you got the hang of it.
>And what about muay thai exercises that seem to do the same for the shins?
Wolff's law applies the same to that form of conditioning as well. Though part of it is also about lowering the pain response as well.
>Those videos of legs snapping like rubber are pretty terrifying.
That has less to do with conditioning method and more to do with maters of recovery and technique in most of the cases. Though some are just down to really bad luck, like the one involving that Japaneses pro wrestler that landed just a little off from a jump.
Generally it is just one of those very small but very real risk of full contact combat sports, or any physical sport know that I think about it. It is very rare though. Like people who break their spine and end up wheelchair bound due to football.