'Chemical imbalance' as a causal explanation seems to me almost useless because every severe change in behaviour pretty much by definition corresponds to change in the brain. How else is it supposed to be manifest? Is there supposed to be drastic change in psychology without change in the brain?

There can be a company pouring lead into a river, psychological hazard during a civil war, and it will always manifest in the brain, what kind of explanation is this? Of course these things can be found in the brain, it would be crazy if they didn't.

That's not to say there's no diseases that are actual individual pathologies of the brain of course, but to use 'chemical imbalance' as a sort of first order response to what may have actual social or environmental reasons say, is entirely circular reasoning.