I think new Hubbers might be too shy to say negative things, and more established Hubbers probably take the same kind of attitude I do:
1. if a Hub is really bad, I won't tell the Hubber, I'll just flag it and it will disappear.
2. if a Hub is bad but can be saved, I'll leave a comment suggesting improvements - but I'll always finish it by telling them to delete the comment once they've read it, as I don't mean it for public consumption (some people send the Hubber an email instead, but I prefer not to reveal my email address).
3. If it's an average Hub and there's nothing nice I can say about it, I won't say anything.
Finally, of course, Hubbers control what comments appear on their Hubs, and some probably just deny anything negative.