Herbs are the initial source of most modern drugs available today. There is an active and continuing scientific research to study the medicinal properties of various plants. Thousands of scientists worldwide are engaged in this research.
Herbal medicine operates on a principle that is fundamentally different from that of modern medicine.
a) Plants possess only tiny quantities of bioactive medicinal molecules. To set off this deficiency, multiple herbs are used in herbal formulations to bring about healing via synergistic action.
b) Plants show variability in medicinal efficacy because of soil, climate, and genetic variations. Synergistic action by multiple plants to a degree is able to offset this too.
c) The availability of quality herbs is a huge challenge with demand exceeding supply.
Herbal medicine will be effective in healing provided, the medications are prepared using quality ingredients. It is here that herbal medicine is unable to compete with modern medicine. There are just not enough herbs available in the world. Till humans are unable to find a workaround these bottlenecks herbal medicine will continue to play a complementary role to modern medicine.
This is a profound question and this answer is just a preview. There is so much more to the subject.
To read more about herbs, herbal medicine, and related subjects, you can check here.