Warning, prepare for a generic-like response~ XD
Nu, herbs are bad. Not so much, herbs are bad, but herbs aren't good. You see, the issue is that when you work with drugs, manufacturers try to replicate a pattern for consistant product results, so that their product is reliable. That's what gives them their reputation, so that's how they'll work to sell their product.
When herbal sources work, they instead emphasize that the plant is healthy and natural. Their reputation is that they have a functional product that is as natural as possible. However, like humans, or any creature for that matter, plant structure can be a little inconsistent in its development. Even carefully honed, plants won't grow the exact same way every time. It's just how living things are.
What this means is that herbs are inconsistent. Debatably it might be dangerous, but I personally have not really heard anything in terms of how breast treatment medication can go wrong. The under/over -shooting can be problematic, and can disturb results, though.