My husband's mom has some serious mental issues, which were never diagnosed, because the family preferred to see her as eccentric, rather than mentally ill.  Of course, if she had been diagnosed when she was young (because this was a long-standing condition), who knows to what horrible treatment she would have been subjected....
Re Trump -- I think that the people who knew him well enough to recognize his condition (if even if they were unaware that it was any kind of neurosis) have serious responsibility for allowing his candidacy.