I have read many times in many places over the years that your digestive processes slow down, but apparently they only stop entirely if you allow your stomach to empty entirely. As long as there are solids for your digestive system to work on, it seems to chug along, though less efficiently, due to blood being directed to other places in the body.
It would make sense that simple sugars would be preferable in that scenario as they are more easily digested, but I have also read (and experienced first hand) that we can only consume so many simple sugars over a period of time before nausea becomes a problem. I don't need anything more complicated for a marathon, but I know that I would run into those nausea issues if I had to fuel through a 50 miler with only simple carbs.