Prioritization with ROI is generally thought of as a quantitative analysis. For hard ROI, that is true. For soft ROI, it is anything but true. You have to make a prediction of the utility of the requirement or feature. That predicted utility is based on our expected utility, which is based on your past experiences. Your past experiences are reflected in remembered utility, which is a function of experienced utility. How can you know with certainty, and use that to prioritize requirements or features?