Does shark cartilage really help with knee joint pain?

Doubt. Read wikipedia...
Contaminants. I agree in a number of areas with traditional chinese medicine. The problem at this time is that shark is a predator fish with of high levels of bmaa (neurotoxin), arsenic & methylmercury.
No conclusive eviden. No conclusive evidence in western medicine. However, this is a common part of treatment in chinese medicine. If you go the chinese medicine root, work with a certified specialist in this art.