Flex Fuel aka E85 is gas w/ 85% alcohol. Alcohol is a mild acid and can attack metals and seals that aren't designed for it which is why older cars shouldn't use it. Most gas sold today is E10 and cars are designed to tolerate if. Alcohol has also less energy so mpg suffers in proportion to the amount of alcohol it the gas.

As far as your fuel pump goes, if it's not E85 rated, it will eventually have problems if E85 used.

On a personal note, I quit using E10 in all of my small gas engine tools because I got tired of replacing the carburetors even on those supposedly qualified for E10. They are all running strong ever since. The downside is pure gas is usually only available in Premium grade although some dealers have it in Mid grade now.