We don't instantly dismiss, at least I certainly don't. I look for the published records of properly carried out trials, don't find any reputable ones, and then
In this case Burzynski apparently doesn't even have a proper trial protocol, and no credible statistics could result. He's also been at it for quite a long time (30+ years!), much longer than it should take to do some proper testing. Hence.... quack.
Look, you don't get to reverse the burden of proof on treatments, where we should accept any claims unless they've been disproven. There are far too many wacky claims to be able to use that approach, even if it was appropriate. If the proponents of any treatment want it to be labeled as genuine rather than quackery, then carry out proper trials and produce reputable publications. Choosing not to do so suggests that the proponents themselves know that it's quackery.
If someone wants to do testing on the effects of chewing a measured amount of certain roots -- go ahead. I suggest you not smear your submitted papers in goat blood, though, and be careful about dosages if you haven't isolated the compound.