Couldn't you ask the same thing about aspirin or literally any other naturally occurring drug with few side effects?

> And also how do they produce such dramatic effects (generally beneficial when used judiciously) in humans with seemingly very little toxicity?

While toxicity may be low, it's very unlikely we'll find few side effects. The reason we don't know many ill effects of the drugs used in small doses is that we haven't studied them properly yet (meaning at a population level over time).