The problem is that there are only few controlled trials for herbs and supplements, mainly because there is no money to be made because the substances can't be patented. If company A finances a controlled trial, then company B can just copy the claims.

Another problem is that these herbal cures may work initially (through placebo effect or a real mechanism), and then stop working. However, at that point the word has already been spread. This is especially troubling in the internet age, where people post reviews after one day of trying something.