ADHD has by far the most successful medical treatment of any mental illness. Something like 90% of cases get positive response out of medication, with 35-40% of people having all their symptoms handled.
My understanding is that for treatment of clinical depression, medication helps around 75% of cases, but that most people continue to have symptoms even when under medication due to the nature of the illness.
In both cases, though, treatment is understood to be a continuous process. You cannot be cured of these illnesses, you can only cope with the side effects.
The treatments are like a prosthetic leg: No matter how much you use it, removing it will bring you back to square one.
There's a lot of research showing the positive effects of medication (and in ADHD's case, the futility of non-medication-based treatments), but there's still a major fight for acknowledging the validity of this form of treatment. Major parts of the population do not think these illnesses are even real!
But it's all pretty dangerous. We have some understanding of how brains work around these illnesses thanks to the research gone into it, but there's a lot of complex interactions going on. Not that physical medicine is much different.