What is the most important factor to consider when evaluating viewability and ad position performance?

The most important factor is that viewability and ad position data are measurable. Without reliable measurement, any analysis or optimization efforts would be based on speculation rather than fact. Accurate and consistent measurement allows for data-driven decisions to improve campaign performance. "Whether the position was relative to the size of the screen" is a component of viewability measurement, but not the overarching most important factor. "That you're counting apps and sites equally" is a consideration for consistent reporting, but secondary to the fundamental ability to measure. "That below the fold inventory cannot also be 'viewable'" is incorrect; below-the-fold inventory can indeed be viewable if it enters the user's viewport for the required duration.