well for one, nutshells (like cashews) are caustic. they contain toxic oils called cardol and produces acids, as a by product. they are commonly used in cleaning products and other industrial applications.
the primary content in wood is cellulose and lignin.
Well not to be argumentative but BFD-Cashews nuts are poisonous until they're roasted what does that have to do with anything?
All wood contains some small of oils and acid-all woods having different properties means some are better suited for smoking but omnly a very few are totally unsuitable-Rhododendron comes to mind.
we are talking about smoking purposes. the components in nutshells have a higher acidity level than wood.
therefore, the smoke produced by them would translate to a more bitter flavour. also the smoke created by nutshells doesn't necessarily carry the same flavour, as the nut itself.
this is just my opinion. if you like smoking fish with nutshells, all the power to you.