I'd say you judge the success of a portrait not on its accuracy, but in its resonance. The two may or may not be mutually inclusive. Think of the Karsh portrait of Churchill, after the cigar snatch - it carries a very potent sense of Churchill, which only tells you ONE thing about the man - far from a complete picture, or even the most dominant thing about him. Ditto the famous portrait of JP Morgan with the "dagger" chair-arm, or the Arnold Newman photo of Krupp, lit with "monster-lighting". Or the Avedon portrait of the Duke and Mrs. Simpson right after he told them he just ran over a dog on the way to their sitting. You look at any of those portraits and they hit you. Or even the Migrant Mother above - it's a powerful portrait, but the accuracy of it is questionable. Dorothea Lange captioned the photo with some exaggerated/inaccurate information to tell a story about rural poverty that didn't necessarily apply to the woman in the photo. Doesn't lessen the impact of it, and the emotional truth it captures. But historical truth? not so much, and to a great degree irrelevant.