This is anecdotal, but just wanted to give an example that I've personally experienced. We used to race go karts with Rotax engines, and the local series organ

This is anecdotal, but just wanted to give an example that I've personally experienced. We used to race go karts with Rotax engines, and the local series organizers decided to mandate 91 octane pump gas ("cost saving measure") as opposed to the VP racing fuel that the engine was designed to use. The end result was that all the racers ended up having excessive carbon buildup in the cylinder, and would go through several sets of the fuel pump internals every year. The fuel pump is a pulse driven flexible element that the pump gas (containing ethanol) would just destroy. Never had those issues with the ethanol free VP fuel.