They might be that strong all right. I know a few woodworkers that bought lots of clamps like that and will use a half dozen or so in a clamp-up. This doesn't make good economic sense to me since you can buy 2 decent quality F clamps for the price of one of those. To me the only advantage of using these is the one-handed aspect but that is an important one at times.
You can get this type clamp in every quality/price range there is and the cheapest ones will break if you tighten them as hard as you can. If you just use them to hold something together while you put a better clamp on it then most of them, including the Magnum's that Dan linked to for $10, will work just fine.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.