As a Hunter main, they're not. To even use Firebolt Grenades effectively (they're probably one of the worst Grenades in the game), you're giving up 2 great Sunsinger perks (increased Radiance time and an extra Grenade) for the sake of using a grenade that can only kill low-Armor builds. It just doesn't seem worth it in PvP. If I ever used a Sunsinger, I'd much rather use double Fusion Grenades - they're much more powerful than Firebolts if you don't give up the perks.