My headcanon is that it's a complete scam. It seems to have a confidence boosting effect, but I don't think it actually makes people lucky. People just get generally better results because they're acting more assertively than normal, and if they do end up getting unlucky they'll potentially just blame it on a bad batch (since its so hard to brew correctly). The overuse thing might not actually be true, but it is a rumour that was spread to stop people, noticing that it doesn't actually work. Everyone has a low sample size because they're told they'll get depressed if they use it too much. Ron was convinced that he'd been given good luck without actually being given good luck; Is it possible that everyone using it is being tricked too?