Are teeth whiteners good or bad for you teeth?

No matter how much I brush my teeth, they still seem to be yellowish. I was told that teeth aren't really white anyway. But that's besides the point. So, I have this fear of teeth whiteners because I believe that they shave off coats of your enamel just to achieve whiteness. But, I want to use them because I think they will be effective. I don't know what to think. Are whiteners good or bad?