I know that is a common belief. I'm not sure why.

I've never seen ( or heard ) of a vehicle's warranty being voided from ethanol. I talked to Ford higher-ups more than a year ago about putting all the way up to E85 into a FEH. Above about 50% ethanol it will trip fault codes, as the data from the O2 sensors may go out of range. Also, above about 50% the fuel injectors may reach maximum duty cycles, and trip fault codes, give check engine lights, and may, ask you to "stop safely now" on the message center. In the rarest of cases, it ( the software ) may shut the car down.

Ford will not pay for re-setting the fault codes, and/or draining the E85 out of your tank if you accidently put it in. So you may have to foot the bills for the cost to "reset" your car back to normal. No warranty was voided in any of the ( two ) cases I heard of when E85 was put into a Ford Escape Hybrid.

The claims about rust & corroision are unwarranted.
A small amount of corrosion inhibitor is added to the ethanol when it is still 100% or 200 proof. The allowable combined acid content is limited to 0.007% and that is for all acids combined, mostly acetic acid, which is the same as vinegar, and comes from trivial amounts of bacteria that live in the corn before the alcohol is distilled. Trace amounts of sulfuric acid may be present, but in parts per million. The pH of ethanol is very close to 7.0. The governmental mandate is ethanol can never contain more than 1.2% water at the pump in the U.S. I think the limit is less in Canada, 0.8%, but I'm not positive, other than Canada accepts less. Ethanol distilleries always keep the water to below 0.75% so there's a little room to pick up water along the distribution process. Ask any good mechanic what 1% water will do to your car. He'll probably say "nothing" or point out there's probably more than that in your gas tank now. Ethanol has been used for over 100 years to remove water from gas tanks, without the need to drain them. Ethanol dissolves the water, and passes it harmlessly through your engine and out the tailpipe.

Very high concentrations of ethanol ( like E85 ) may dry out rubber and some plastics. Gasoline is a thin oil, and a lubricant, where ethanol is not.
But having a 50/50 mix or less keeps the "oily" properties, and will not readily dry out rubber and plastics. The rubber and plastics that are not compatible with high amounts of ethanol were phased out in the early 80's, and some states have been using ethanol at the pump for more than 25 years already. How many engine complaints from ethanol have you heard about over the past 25 years?

Ethanol will lower MPG, but not so dramatically as once thought.
I have measured no loss in horsepower, or performance, or response from the engine with 30% ethanol in my FEH. MPG is lower, but about the same as 10% ethanol.

I bought E30 for $2.54 last week, and lost about 12-15% MPG tops.

-John