Are grain-free dog treats safer than treats with grains?

Not necessarily. Veterinary discussions referenced in r/AskVet and r/DogAdvice point to the FDA's ongoing investigation linking some grain-free diets to dilated cardiomyopathy (DCM). Treats make up a much smaller share of calories than food, so the risk is lower, but vets quoted in those threads generally do not consider 'grain-free' a safety feature on its own.