My opinion does not matter here, only that of the admins and mods does.
That being said, the standard I apply is "What is the obvious target of the harsh words?" If the target is actual actions or expressed ideas, harsh away, but provide rational support for the criticism. If the target is the person himself, lay off, even if you think you can rationally justify the adjectives.
Oh, and never, ever say "Your an idiot." It leaves the reader with zero doubt as to who the idiot is!