Ethanol ruins your engine if it is in too long. I just saw an engine that sat too long with four stuck intake valves.
The alcohol dissolves gasoline varnish in the fuel tank. It eats real rubber fuel system parts. Neoprene has been used instead of rubber for decades.
If anything stuck the valves, it was the aforementioned gunk, or water suspended in the fuel.
Ethanol itself is not corrosive.