I have always thought that the main problem of Sitney's classification is that is too disperse, it doesn't follow a single line or concept. It mixes criteria of content (themes, mythopoeia) with form (editing patterns or physical qualities), so in the end you end up with very different groups and a lot of uncharted 'space' between them. The definition of structural film is built on some arbitrary elements, instead of trying to create a proper academic definition of it.

So of course, you can have narrative content and at the same time follow a structure, or with a different example, you can have films both poetic and abstract. Which one is the main characteristic, the predominant one? The same problem exists with that too broad category of 'found footage' films...