Surprisingly, I don't think the issue with potatoes is so much energy intensity as it is that the starch content of potatoes is a lot lower than it is for corn and wheat. So I think it's less an issue of energy intensity and more an issue of feedstock supply and volume. It takes a lot more potatoes to make a litre of ethanol than it does corn or wheat. You need to have a lot of them available, year in and year out, in order to make that work.
That would be a risk factor associated with using potatoes. What happens if there's a bad potato crop? What happens if there's a very good potato crop, in terms of quality and the amount of waste potato that is reduced in that year? What are you going to do in terms of the feedstock for that one year in five where that might be the situation? I think that's more the issue for people who have looked at it: the risk associated with feedstock supply.