Grappling is dangerous for the knees.
Freestyle wrestling is bad because shooting for doubles and singles puts pressure on your knees, and for example when someone has you in a low single and is pushing you sideways to bring you to the ground. That's absolutely terrible for the knee.
Judo because if you are not careful you will fall with your knees in bad angles when throws get messed up. Also, techniques like tai otoshi can easily blow a knee.
In BJJ because it induces alot of unnatural torsion of the knee. For example, pulling your leg while executing a triangle can be bad for your knee, depending on your flexibility. Or escaping stuff like DLR guards, etc.
I won't even go into catch wrestling or LL where toe holds are used.
All in all, just look at the amount of injured knees anywhere near tatamis and mats. Grapplers must accept that underlying danger.