My opinion is that saying that all mental illness falls into one camp is oversimplifying. Someone who's schizophrenic is definitely in the brain category, according to the current consensus I've seen. Depression is moving into that camp. Anxiety is on the fence- it can be chemical or mental.

If I were to answer "is mental illness a mind thing or a brain thing?" my answer would be "neither, both, one, or the other" because the brain is a complex thing and breaks in a lot of different ways.

Anxiety, for instance, is typically treated with temporary medication and long-term therapy. We treat it as "mostly mind, hint of brain."

Depression is treated with long-term therapy AND medication, or just medication. It can be a product of thinking patterns, but the consensus now seems to be that it's mostly a hormone thing.

It feels, to me, like if two groups were arguing "grass is yellow" or "grass is blue" when most grass is green but there are weird variants that are yellow or blue.