I've never use the shells only. It sounds like they may work well as a smoking medium. However, I do remember some years ago when I had a batch of pecans that had grown old (had my own trees). So, I decided that I would use the old pecans in my next smoke (a whole ham). I was using an ECB back then. I poured the pecans in on top of a good bed of charcoal. The smoke was thick and dark. And the meat came out with a bitter taste. At first I thought that the taste was a result of my having used the whole pecans. That was probably true. However, there is also the possibility that I was very new to the smoking effort back then and may have generated a perfect combinition for the formulation of creosote. I really don't know.
But after all this wondering around, I'd say go ahead with the pecan shells. Maintain good ventillation (full open with the chimney vent) and hopefully all wil go well. Just be aware that whole pecans may render the whole product bitter.