I don't know if this helps, but I regard "mental illness" as a social/cultural construct. Yes, there are some things we call mental illness that would probably be considered mental illness (or the equivalent category space) in any culture/society, but it gets really fuzzy around the edges and some things Western culture considers mental illness wouldn't be thought of that way in other cultures, vice versa I believe some patterns of behaviour/belief that are thought of as normal in Western culture would be regarded as mental illnesses in other cultures/societies.

I believe this based on my own experience with mental illness (which I of course think wouldn't be, in a different culture), and by analogy with physical illnesses. Anyway, it makes perfect sense to me that "some spiritual experiences are treated as signs of mental illness" - it's all a matter of whose definition of "mental illness" is in play.