#1 availability. good luck finding it at a random gas station if you street the car.
#2 make sure your ENTIRE fuel system is prepped for e85.. its highy corrosive and eats typical rubber lines etc. Need to make sure everything in your car is e85 safe including fuel pump, all lines and even your injectors/o-rings and everything in between.
#3 tune. iirc, it takes 33% more e85 to produce the equivalent power as gasoline, meaning your injectors have got to be much larger than would typically required for your power goals with gasoline. plus larger injectors can sometimes cause idle problems but really depends on your tune.

If i were to want to run e85 i would most def have a ems that allows for dual maps so i could switch to 93octane if i had to in a jam.