The real danger is that there will be three fuels on the field when there used to be only two. Accidentally using 100LL in place of G100UL or vice versa isn’t as much of a concern as accidentally switching Avgas and Jet A.
I am bilingual and speak both English and Spanish interchangeably. Typically I speak English with most people, but if they are not an English speaker, my brain automatically switches to Spanish. My brain (functionally) is accustomed to a world with only two languages. When I travel abroad (usually France), my brain still operates the same way. I know the French don’t speak Spanish, but It is very difficult for me to force myself not to speak Spanish when I am in France because my brain is wired to think that if someone doesn’t speak English, they speak the “other” language.
I would be concerned about the same situation arising on the field. “The pilot said not to use 100LL, he uses the other stuff”, or any other similar thought process could lead to someone inadvertently using the wrong fuel between Avgas and Jet A. I know when I taxi up to the pumps at an unfamiliar field, if I see two pumps, and one says Jet A, I will be using “the other one”.
People need to be made aware and train themselves that there are now three choices; to be very specific and clear about what they are dispensing, what is acceptable (or legal - if they don’t have the STC) for use in their aircraft, and whether that fuel is in this tank, that tank, the truck, or a different fuel farm on the other side of the field, etc.
No different than when I was taught by my instructor that I want blue, dry fuel and not clear, slippery fuel - we need to make certain that we train ourselves not to make refueling mistakes. It’s not fearmongering, its simply being smart.