Yes, of course, differential diagnoses can be challenging. But I am talking about the classifications, not the patients (the classifications are fairly straightforward but the patients aren’t, in most cases!).
Many times we tell a person that they have a “Personality Disorder” without telling them what it is specifically. This is necessary in order to facilitate treatment. It is especially true in narcissistic disorders. Never tell a narcissist that they are a narcissist!