I don't think "natural" cures are completely ineffective.... mostly because the flip side of that is assuming that all "natural" supplements, etc. are safe because they're "ineffective". Nothing irritates me more than seeing a commercial toting an herbal/etc product that's "PERFECTLY SAFE because it's ALL NATURAL!!!" Know what else is "natural" and "plant-derived"? Ricin (castor beans), cyanide (found in small amounts in almonds and apple seeds), atropine (jimsonweed), digoxin (foxglove), etc etc, all of which will kill you in fairly small doses. Anybody who doubts the power of certain plant compounds need only look to the world of illegal drugs... where do you think cocaine comes from?
I do feel that because of their complexity, those herbs that are biologically active are not going to be as targeted as drugs that have been tweaked in the lab. For example, atropine has a lot of medical uses, but you don't see doctors going and feeding people jimsonweed tea because there are a whole lot of nasties that come along with that... plants don't often manufacture just one toxin. Dosage is also very hard to control, since the amount of a compound in a plant is dependent on growing conditions (as growers of a particular herb will be quick to inform you). BUT the natural world is an excellent place to look for these compounds... nature can produce some pretty wild molecules that either can't be re-created in a lab or would be extremely difficult to do so. Sometimes all modern medicine is, is old cures whose active properties have been ferreted out and distilled into a more effective form. The most famous of all? Aspirin, derived from salicylic acid, named for its source- Salix, or willow trees.
Still... because of a lack of research, there are a LOT of so-called "natural cures" that do a) nothing or b) more harm than good. Colloidal silver is a prime example of this... first off, anything toted as a cure-all like that is an obvious fake, because there's simply no such thing. Silver has its place- it's found in assorted topical creams and cauterization compounds for its mild antiseptic properties to the best of my knowledge- but INGESTING a heavy metal is never a good idea, because it's never eliminated from the body, hence the blue man. You wouldn't attempt colloidal mercury, no? Same family...