It seems to me that any burden of proof would lie with the manufacturer, who must convince the consumer. When one consumer says "it works" and another consumer

It seems to me that any burden of proof would lie with the manufacturer, who must convince the consumer. When one consumer says "it works" and another consumer says "it doesn't work", then we're outside the usual burden-of-proof social dictates and we have to adopt new rules. Such as, I propose, both people need to bring evidence or expect to convince no one.