I can see the logic behind the cancer theory, but I would think that the programming for a total bodily reset with every Change would pretty much obviate incipient cancers (in the same way as I think it would stop prion disease in its tracks - no worries about eating cattle in the UK, unless the owner of the cattle catches you).
I mentioned parasites in another post...*not* sure at all that the Change would kill off, say, tapeworm eggs or the like. That could be really nasty.
I'd also have to wonder if werewolves, with their very powerful immune systems, aren't occasionally prone to severe auto-immune diseases. Although maybe keeping a low parasite load going at least diminishes the likelihood of major allergies (helminth therapy - yes, it is a real thing, although I suspect it will remain a difficult sell to the general public). But I could see a werewolf community in which, occasionally, pack members do go down with things like massive rheumatoid arthritis (I'll spare the "lupus" jokes, since it really is not even slightly a funny disease). In fact, if one's world includes a huge physical risk to the Change beyond the initial injury, one obvious mechanism would be a total immune reaction or cytokine storm as a response to the change in various antibodies from human to lupine...could lose any proportion of potential changers one cared to, possibly with dramatic and documentable symptoms such as those of the 1918 flu.