For the most part, the appropriate dose of skeptical salt has been added to this thread by other posters (pleasant surprise!).
Short answer: no one knows. The items called “mental illness” are poorly defined and poorly understood.
Political codicil: the fact that no one knows is obscured in part by the economic politics of the pharmaceutical industry which would like us all to believe that their wares work with far more specificity and reliability, and far fewer undesirable general effects and side effects, than is actually the case; in part by the social politics of involuntary treatment, because it is easier to defend imposing unwanted treatment on people if you can declare that you know with certainty that their disturbing behavior is caused by a chemical imbalance that will be fixed (with specificity and reliability and few side effects, blah blah blah) as soon as we shoot them up with the latest WonderDrug; and in part by the interdisciplinary politics of the field of medicine, which deserves a paragraph of its own.
The field of medicine is, as you know, subdivided into subfields such as neurology, endocrinology, gastroenterology, and so forth, and psychiatry is theoretically among them, but for reasons historical and social, not entirely, not quite. In the vicinity of a century ago, a great many human ills were not yet fully understood, but the hope was that the shining beacon of scientific inquiry would make them yield up their secrets; and the general disease-model they worked from made them think in terms of every ailment (symptom) being due to a specific and different disease which would be an infection, inflammation, or disordering of a tissue or organ, and the right medical intervention would address this and cause a cure. Problem was, mental health was being dealt with as a social problem rather than a medical one, as “madness” always had been since long before modern medicine, and in practice the doctors who were hired to be in charge of mental institutions were hired more to convey authority and gravity than to do treatment and research. The first major breakthroughs in understanding “mental illnesses” in conventional medical-model terms didn’t come from within psychiatry but from elsewhere: one dementia turned out to be latter-stage syphilis; another became understood as part of the pattern of epilepsy. What happened was that these diseases, now understood, were taken out of the realm of psychiatry – you don’t go to a psychiatrist to get your epilepsy or your syphilis treated – and over time what remained defined as “mental illness” were those things that had never yielded up solid etiological explanations. And the other branches of medicine continue to view psychiatry, not without a fair amount of justification, as pseudo-medicine. Within the understandably defensive field of psychiatry, there is a certain tone of “I am not a quack, look, we treat specific medical ailments which are scientifically known to be chemical imbalances of the brain and we know what causes them and we can fix them with our medicines, see?”
But it simply isn’t the case. Compared to how we understand schizophrenia and the other “mental illnesses”, our understanding of AIDS, cancer, Alzheimer’s disease, and other medical problems considered to be modern challenges for the medical field looks complete and rigorous in detail.