It took a lot of time and money to prove otherwise. Herbal remedies and many folk remedies have not survived even the weak scientific testing that researchers used in the 1920s, much less today's standards of study. If and when they do, they stop being alternative medicines and become part of our scientific therapies. If a person believes in a remedy, and if that remedy is truly harmless, then using it will improve most conditions by about a third, and clear some up completely. Unfortunately, we have reason to believe that they're not all inactive.
Plants produce a wide variety of complex chemicals that mimic substances use by our own bodies.
Many powerful medicines, poisons, and hallucinogens have been derived from plants. Evolution seems to have a somewhat limited range of designs, and tends to reuse molecular shapes in different species for different purposes. These substances have every imagineable effect, from hallucinations to sedation, from quick death violent vomiting. Today's herbal remedies are not likely to have obvious seriously harmful effects. Even though they're completely unregulated and largely untested, that kind of obvious harm would get publicity.