Are clay courts harder on the knees than hard courts and is Rafael Nadal stupid for blaming his knees on hard?

My brother is a huge Roger Federer fan and his accomplishments aren't enough for him that he has to berate Nadal for being whiny. He says that a firmer surface effects your knees less than an unstable surface (I almost believed him for a second recalling a drill our coach made us do jumping in a mat that indoor long jumpers go into and it was much harder than jumping on the courts but if he was right that would mean virtually the entire tennis community are idiots) and the reason why Nadal has knee problems is because he insists on playing all of the clay tournaments. He claims there is some physics behind this. Anyone who knows physics is that right? Also my bro makes fun of Nadal for quiting when he's losing or faking an injury much like Djokovic (he references the last two Australian opens and the French Open where he lost to Soderling). Feel free to comment on that as well