The problem is that "mental illness" is subject to abusive definition at the hands of an oppressive government (see: the history of any communist autocracy, where mere political opposition was adduced as evidence of "mental illness").
Our present Federal government is not to be trusted in this respect, and I would include many members of both parties in that group. But local authorities in large measure still can be trusted, and that is the level at which such decisions ought to be made.
Yes, I agree.
With Obamacare, the government can declare ANYONE mentally ill, and seize your firearms.