You should tell them why you are not satisfied, because some of them will have reasonable explanations for what they've done. Those reasons might be good ones, even if you have equally good reasons not to go along with them. Other people need to know that they missed multiple errors or they overstepped the boundaries of their task. Dan pointed that out above. I took a lot of proofreading tests back in the day, and not getting the results was a drag because how could I improve if I didn't know what I had missed? Tell them, briefly.
Overstepping is an issue I've encountered a lot (and I did my share of it as a proofreader, which is why I stopped proofreading). For instance, a proofreader should not change my prose, but some do, anyway. Meanwhile, they miss a dropped end quote mark or an extra space that should be eliminated. A copy editor will change something that isn't grammatically incorrect simply because s/he doesn't like my choice of words. Again, I don't consider that doing the job properly. Recently, an editor improved the grammar of one of my characters, which I considered an error in tone because my character was relatively uneducated. The attempted change highlighted that I probably needed to rewrite some sentences. Then again, maybe not. It did add work for me, just as the other bits of overstepping add work.
If the sample really was egregiously bad, you'll be doing the person a favor by gently letting them know, instance by instance, what was wrong. Maybe you don't want to. That's fine, too.
