I find that many of the nonfiction writers I work with are maybe just a little too smart for their own good. I say this with affection and appreciation for how hard it is to communicate paragraph after paragraph of complex information and insight in a coherent way.
Many nonfiction writers are experts in their particular field and know far more about it than will most of their readers. They are also extremely close to the material and tend to lose their ability to be objective. Their challenge is to avoid both overestimating and underestimating their readers.
An editor can help a writer accomplish this balancing act only if she is able to wrap her mind completely around complex material and comprehend it well enough so that if certain passages seem to not be understandable, she can be confident it is a problem with how the information is being communicated and not with her own ability to absorb it. I have had a lot of success working with complex books by highly expert writers.
Coherent organization of topics in a nonfiction book is also a major challenge. The human mind does not think in terms of chapters and subheadings, and therefore when writers sit down to write, knowledge and insight often come out in more of a stream-of-consciousness fashion rather than in finished book form. Many extremely intelligent nonfiction writers need help breaking down their topics into appropriate parts, chapters, headings, etc., in order to avoid repeating in Chapter 12 what was already fully covered in Chapter 3. A good editor will spot related concepts and help writers devise a logical narrative flow.