The frag grenade is actually a nerf for VG who would be OP having both smoke grenade amd flame grenade since they would be to deal with any hmg in garrison.
I would use the term "sidegrade". Nerf implies that there is no positive to having a burst damage nade instead of DoT, which I'm sure you will agree is not the case. Of course, not being able to deal with garrisoned HMGs as easily is a negative. A strength balanced out by a weakness; a sidegrade.