Quick history lesson that's causing me pain.
RSS was originally meant to showcase RDF, but in that role it utterly fails. Because it turned into a poor expression of both XML & RDF. Later Atom was standardized to be a more XML-onic RSS but by that time the harm was done. Trying to deal with two formats just makes things in the work I'm doing. Because I have to ignore namespaces which XML-tools don't like to do.
So I'll stop trying to use XPath and build simpler primitives for myself instead.