Well, one major drawback of ethanol, is that a significantly large portion of land-space/crops, that are normally intended for human and livestock consumption, have been replaced with plants-for ethanol production.
Quite literally we are now farming a rather large global portion of land, to fuel machines, rather than humans.
What's worse, is that some of that land set aside for Ethanol production utilizes crops that require large amounts of fertilizer, derived from petroleum/oil (especially in terms of corn based crops, in that corn is especially difficult and tough on the soil, and requires a large amount of fertilizer to grow).
So we're using petroleum/oil, to fertilize lands, that produce Ethanol, which is mixed with petroleum/oil at the refinery and then pumped into vehicle gas tanks.
Oil based products are being used to grow crops that replace a portion of oil based products.
There's quite a bit of a contradiction there.
Wondering why beef prices, and some crop prices have skyrocketed in the last few years? This is part of the reason.
NEXT... as for engine performance, ethanol in oil/petroleum makes it much more difficult to start vehicles in winter months, in northern climates.
As well, greater ethanol content has been shown to cause greater wear/damage to some engines, because it burns much hotter, and corrodes engine components more than standard oil/petroleum (2012 Auto Alliance report).
There are also several reports that show less fuel-mileage and performance with gasoline/ethanol mixed products.
In short, I personally feel that it doesn't make much sense to give up all that farm land, to add ethanol to gasoline. I believe we're paying a MUCH HIGHER price for that ethanol than we suspect.
Ethanol is NOT the answer to the world's energy demands.
Solar energy is the answer.
Solar panels will continue to get cheaper and cheaper, until we reach a point in which large scale solar power generating facilities, combines with many smaller solar cells, will all work together to energize the grid, and recharge electric cars.
Again: solar (and not ethanol) is the answer to many of our energy problems.