The traffic is only unencrypted if the destination traffic would normally be unencrypted. That means that an exit node can't MITM what would already normally be an HTTPS connection. It's a small nitpick grammatically, but the distinction is an important one.

Edit: Another potential concern that I hadn't thought of might be pavki's concern for traffic correlation attacks, but that's entirely speculation on my part since they haven't provided any clarification to their original comment.