The problem I see (regularly) with 'herbal remedies' is that everyone thinks their source of information (book, herbalist, google, whatever) is good.
The same problem exists, btw, with their physicians. At least with physicians there is a prescribed course of study and licensing, which cuts down but doesn't minimize the fraudsters like Mercola. Some people are kind of harmless (my barber in Los Angeles, who was quite proud of his Doctor of Naturopathy mail order
degree certificate and his copy of Materia Medica... always 'diagnosing' me with things I didn't have, and usually didn't exist.
As far as strengths of herbal remedies.....exactly how does one determine how much of a product that has highly variable potency due to growth conditions, preparation, storage conditions, and how it is administered? In a cup of willow bark tea are you getting 325mg (1 standard aspirin tablet) of acetylsalicylic acid, or 3250mg? If you're taking a cup of it every four hours, that makes a bit of difference. And something that most herbalists cannot explain is how whatever they're advocating survives in the pH 1 environment of the stomach, or the pH 12 environment of the duodenum, and gets absorbed.
Then there is the problem of plant identification - plants have different names in different parts of the
country world, and what people think they're taking isn't what they actually get. Usually it's just not effective, sometimes it's quite detrimental. Or the quite earnest but essentially clueless teenager at a 'survivalist' show who was hawking a 'liver cleanse' product that would prevent gallstones. When I asked what it does for people who have already had their gall bladder removed told me that they could still get gallstones (OK, #anatomyfail, I guess).
For most illnesses, the fact is that most people will recover on their own, without any intervention at all - good or bad. Most of what medicine does is shorten (hopefully) the time that people are uncomfortable, but all antibiotics do, for example, is slow down the growth of bacteria so the body can heal itself. That doesn't mean they are useless, sometimes the body can't keep up with the insult to it, but it does explain why most unconventional medicine has the appearance of success: Tincture of time, as Osler said, is what cures most everything.