Supposedly, cars from 2000-forward can run E15 without harm, per US EPA (but some mfgs still advise against it, even with their new products. I know Honda does.) EPA may actually go farther back than 2000 for compatibility; I forget, TBH.
I always choose E10 over E15, myself. Unless I've traveled somewhere that actually carries 100% gasoline, and then I'll get that, and see an almost immediate improvement in MPG. As Moose alluded to earlier, one thing ethanol *does* do is raise octane, although it doesn't have the same energy output as gasoline. (Nothing does, btw - gasoline contains more energy per gallon than any other fuel.)
For our GM trucks, 'flex-fuel' mostly means two things - injectors with a variable rate, and two fuel maps in the ECM. If those two things are accounted for, via replacement injectors and a tune, then most of us could make our trucks E85-capable. Limequat turned it on for me when I sent the old Sierra's PCM to him. When the PCM detects the change in fuel type, it changes the flow rate on the injectors within a few miles (increases for E85, decreases back when E0-E10 is put back into the tank). The other variable is rubber components - ethanol is hard on rubber. But if it's not part of the fuel delivery system, there's not much else to worry about.
Corn ethanol (what we use here in the US) is 'energy negative' -- it takes more energy to make it, than is produced from it. But midwestern farmers like it (and a sh!t ton of infrastructure was built to make it), so we continue to use it. At least for the next several years, while the fleet converts to ZEV.
In Brazil, they make ethanol from sugarcane, which isn't as energy intensive, and doesn't rob a feedstock (most corn grown in the US goes to feed livestock, not people). A bit more difficult to grow sugarcane here (and it may soon be more difficult to grow corn, as well - the areas that have historically grown it have been getting pelted with excess rain the last couple of years, which affects both planting date and yield.)
