As for AFA I have a very good Canadian friend that sent in his toy fair Vader to AFA. He's an excellent collector and a very big vintage carded collector as well so he's quite aware of grading and quality factors for cards, bubbles, etc. He was pretty convinced he sent in something that should come back as AFA90 and it came back AFA75. He is absolutely convinced that AFA switched out his Vader for another. Now, it's completely and utterly anecdotal and there is precisely zero proof behind the story. But I know the guy, he's been collecting since he was a kid and that's when the movies first came out. He's a knowledgeable authority IMO and he thinks he was screwed. I'm betting he was right.
Unfortunately, this isn't the first time I've heard a story like that. Others who have made the accusation usually get "it must have been damaged in shipping it to us" and you really have no way to prove anything.
The only time I've ever considered grading anything was always when it was an in-person type thing where you're there to watch them.