There is a suspicion that marijuana smoking can also increase the risk of lung cancer, but as you pointed out, it's very difficult to provide evidence for that because people smoke different types of marijuana and the number of joints they smoke also varies from day to day. It's very hard to quantify the amount they smoke in comparison with something like the number of cigarettes.
I have bronchoscoped a number of people who smoke marijuana. They have tremendous inflammation in their bronchial tubes and it leads me to think that they must have caused damage to promote lung cancer.
This is something we need to do more research on to decipher the exact problem with long-term smoking of marijuana.