If you’re expecting Freddy to be pretty, it’d be “based on what I’ve heard from other people, I can fairly reasonably draw the conclusion that Freddy is pretty” – the context being that you’re saying things about Freddy without having met him. If you’d met Freddy, and you think he’s pretty yourself, you’s be using と思う. Possibly a better way of defining the usage would be “based on some reliable evidence”.
It’s not really that it sounds awkward, it just makes you sound over-confident. I think. If you said in English “I expect I’ll win the lottery tomorrow”, you’ll either come across as arrogant or delusional. Everyone’s going to look at you and go “… Based on what evidence?”