I’ve seen too many of my friends die after receiving massive doses of chemo to become a critic of anyone who decides not to undergo chemo in favor of more experimental or holistic treatments. I once read a doctor’s quote that “some people survive cancer despite chemo.”
You’re probably right that childhood cancers have a much greater cure rate because of chemo, but you have to admit the side effects can be so terrible that some adults, understandably, decide to forego it.
As far as comparing chemo truthers to anti-vaxxers, people refusing chemo are making a personal decision about their health that affects no one else. But it could be argued that people who refuse to vaccinate are affecting public health at large. There’s a major difference.