Well, everything can be harmful in certain quantities. There are oils and acids in peanuts as well as pratically all nuts that can lead to canker sores (I'm not sure about cold sores) if ingested on a frequent basis. But it's not the only factor, though it may be a major cause. Even sheer stress can cause canker/cold sores.

And it depends on the person. Obviously if you're alergic to nuts, peanut butter is not for you. However, I still enjoy it occasionally, but I'm more of a creamy than crunchy person. It's annoying trying to dig the nuts out from in between my teeth

I will add, though, that eating too many nuts in succession will often leave my tongue raw, mainly because I'm a huge fan of almonds and I have a strange habit of having to peel the skin of the almonds off with my teeth before eating them slowly. The whole process takes about 10 minutes per almond But I can get the same feeling from too much salt. It just depends on the quantity, and I know when to stop.