Different forms of ethanol have different impacts.
Ethanol from crops that would have been burned creates a net zero GHG impact, especially that from cane sugar production.
Ethanol from actual corn, due to the high water and fertilizer content, and the fact it is diverted from feed crops for animals and humans, has a high GHG impact, and actually costs more to turn into ethanol, in terms of GHG impact, than if used for feed.
Ethanol from switchgrass and blue-green algae is fairly good, with a net positive GHG impact. Switchgrass is a rotation crop, so it has very low fertilizer and water needs, and converts fairly well. Algae does have high water requirements, but if produced in areas with high water content, or used as a filtration crop to clean polluted water, can be a net positive.
But corn ethanol ... that is a bad choice.
(caveat - I invested in various corn ethanol IPOs at various times)