I think anyone who is in this field realizes that the etiology of mental illnesses (including depression) are multifactorial and much more complicated than we actually view/understand them currently. The term "chemical imbalance" is a misnomer and is so simplified an explanation it might as well be false. Yes, of course their are strong correlations among serotonin and cortisol in depression, and yes, medicines that act on serotonergic systems (among many other mind you) seem to relieve symptoms of depression. But, I think any reasonable person can see that there is probably alot more going on here, both biologically, and of course, SOCIALLY.
Although mental illness is as important to treat as any other disease, I would say NO, its not the same thing as diabetes, cancer etc. As of today, psychiatric illnesses are all syndromes. Some have associated biological abnormalities that we have observed, and some dont. The dysregularization of the HPA axis system in chronic depressives, a variety of structural brain abnormalities in schizophrenia, etc. But certainly nothing we have pinned downed as "THE PATHOLOGY" behind any of the psychiatric illnesses. And there probably wont be any ONE pathology that explains all the symptoms of bipolar or schizophrenia. Its multifactorial. Multiple brains systems, multiple dysfunctions that all result and/or influence the other, and multiple environmental influences that shape the course of the illness and how symptoms are expressed.