It does terrible things to the evap system and that includes the charcoal canister. The canister in newer cars is not vented to the atmosphere like the older cars, so once saturated with raw gas, it doesn’t clear after a few miles, it stays saturated for a while. This, of course, causes problems in the evap system and never ending ‘check engine’ codes. Other problems are raw gas mucking up evap sensors and valves, requiring them to be replaced. And some are not cheap. The evap system in newer cars is designed to sacrifice itself to prevent fuel spills from over-filling, so the constant punishment for over-topping the gas tank could lead to expensive repairs.