I think chemical imbalance can certainly be a factor, the brain makes lots of chemicals that effect all kinds of bodily and mental functions so I don't see why something couldn't go wrong there under the right circumstances. But its a factor then there are other factors like environmental, biological and social I imagine those factors can influence how ones brain develops and what issues might arise.

As for anti-depressants or any other medications, all they can do is treat symptoms..I think they get pushed as a cure all and are over-prescribed probably due to the pharmacutical industry. But they do have their place....and of course everyones brain is different so the same medication won't help everyone with the same mental disorder, especially since a lot of people have different combinations of disorders as well.

I also think its possible different factors can cause the same disorder, like maybe not everyone with depression has it due to the same factors, so not sure they even would be able to isolate specific causes of each mental disorder that could be clearly seen on a brain scan for instance. From what I've learned with my own reading and taking psychology before i dropped out of college mental disorders are very complex.