Making a positive claim (the latter) only sets yourself up to be discredited when, after some amount of time, one of the products inevitably is proven to work

Making a positive claim (the latter) only sets yourself up to be discredited when, after some amount of time, one of the products inevitably is proven to work for something. It's better to make the negative claim (the former) rather than risk losing what little authority you have with people who are struggling to understand the evidence behind these products to begin with.