The problem isn't so much the energy density as the fact that ethanol has some nasty reactive propeties part of which is because it's hydroscopic and part is that fuel systems weren't designed to encounter it. This has been a big concern in the boating industry where WATER is a more invasive problem. The sad truth is aviation fuel tends to sit in tanks a lot (avgas is specifically stabilized) which car gas is not and it gives the ethanol time to attack the tanks, etc...