The braces are only one example of this: there have been many times when I said I wasn't going to do something because other people said that it was really bad, and the people almost always backtrack immediately and tell me that it's really not that bad. I understand that you wouldn't want someone to follow you blindly or make a decision based only on your experience. But when someone is taking what you say seriously, you don't have to back out; you can explain the specific reasons that you didn't like a particular experience so that the person can decide whether or not they would be bothered by the same things. You can also explain how your situation may be different from theirs.
If a person wants something so much that they're willing to do anything to get it, they won't be fazed by anything negative that you tell them. If they are fazed by something you've said, it means that they probably would have had a problem with that issue when they discovered it on their own, whether you had mentioned it or not. If there is any doubt in a person's mind, you would be helping them by bringing those doubts to the table before they begin something that they might not want to start.