Dermal filler in the frown area (glabella) is one of the most hazardous areas to treat. There are small blood vessels in the frown area that communicate with the eyes and if the filler is accidentally injected into a blood vessel it can result in blindness and also necrosis of the skin on the forehead.
Some very experienced injectors treat this area with filler but the injector has to be very confident with anatomy and very confident at managing any complications that arise. Personally I feel that surgeons are the best people to treat the frown area with dermal filler.
I feel that anti-wrinkle treatment to the frown area is infinitely safer. It may take more time to reduce deep frown lines but safety needs to be paramount.