The reason why is that there is a cap on charge speed. Perun's Fire gets one of the smallest overall benefits from coils in the game. I have a Light/BEWARE with Accelerated Coils and a Perun's Fire with Accelerated Coils. The Light/BEWARE has an identical charge time and higher Impact, Range, Stability and magazine size. Basically, L/B or Final Rest with coils are superior in every single stat. Using AC on Perun's Fire lowers the charge time by a little over a tenth of a second. Obviously not everyone has another model to max the charge speed but... even then, it's a trade. It's not bad but I'll always use Hammerforged over it on mine; the charge time matters the most for me when I need a 2nd shot but Range makes it OHKO more easily, thus not needing another shot in the first place.