The big problem with knees, is usually people either wear out the cartilage on part of the discs, or they over stretch/pop a tendon. The latter is mostly an issue with shoulder/hip throws, sports with cleated shoes, tackles, or locks on the ankles/knees for grappling.
The former, though, comes from bad posture and... sitting with pressure on the knees.
A lot of physical practices advise to not put your knees further forward than your toes - this isn't just a balance thing, it's also the point at which you're hyper-compressing sections of your meniscus discs. Do it long enough, over time, and the meniscus develops cracks. Then tears. Longer still, you can wear away a whole section and have bone-on-bone grinding.
Unfortunately, this isn't something the body can repair, really. The same thing happens with spinal discs, given enough time.
This is where you end up having to decide what parts you want to practice, and how much. Just as much as full contact sparring is a thing you CAN do, you have to decide at what point you've now just accruing more damage than learning.
Given the formal kneeling stances, there's not a lot of mitigation - it's a knee at the maximum flexion position, there are no muscles to take the weight off of it. Some people use cushions or wedges between their calves and butt to raise their body and reduce some of the direct pressure, but obviously, traditions usually do not look well upon that choice.