If you have an older vehicle/engine, the fuel system was built with rubber and plastic parts designed to be resistant to gasoline only. It may or may not be resistant to ethanol by luck. Newer engines fuel systems are usually resistant to ethanol, especially the "rubber" parts. But, there have been some circumstances where cheap plastic carburetors have swelled over time from ethanol and need to be replaced.

Absolutely fresh ethanol-gasoline mixes in clean engines will run fine, without problems. If you use it quickly and constantly, you will not have problems... But, there are a lot of "ifs and buts".

Ethanol absorbs water, lots of it. It will actually remove small amounts of water from a wet fuel system, if it is fresh. But if you do so, use the fuel up quick!

Here are the problems: Ethanol is constantly absorbing water. If there is water in the bulk tanks at the gas station, it will absorb it. If there is high humidity in the air, it will absorb it, either at the gas station or in your own gas tanks. Over time it will keep absorbing water, until the ethanol-gas mixture becomes saturated and can absorb no more. Now, if it is saturated, and conditions change (such as temperature, evaporation, additives, etc.), water may actually come out of solution and sink to the bottom of your tank, carburetor bowl, or fuel line. Seperated water can cause all kinds of unwanted chemistry inside your fuel system.
A common problem for 2-cycle engine owners' is water in the fuel. If you buy or keep old ethanol-gas mixtures with significant water dissolved in it, you cannot see the water,YET! Now, you add your 2-cycle oil to your gas, and you have changed the chemisty.--Like a magic trick, water drops out and settles to the bottom of your tank. That water will rust the bottom of a metal tank. Since that dirty water is on the bottom, when you hook up your outboard motor to the tank, that is the first liquid into your carburetor. Now your motor won't start. So you put the motor aside in the garage for awhile,--now you will need a carb re-build, if not a whole new carburetor.

When it comes to 2-cycle engines, BUY PURE GAS ONLY!