Gearbox seems adamant that grenade spamming is unintentional and shouldn’t be possible, but honestly, grenades don’t do enough damage for spamming them to be worth it to begin with. Pre-nerf Hex, sure, but after the nerf even the Hex’s damage pales in comparison to that of a decent weapon. And MoD isn’t a problem either thanks to the internal CD; 3 rounds per second is only meaningful on low fire rate weapons like heavies, and since the Flakker adjustments none of those weapons do enough damage for effectively infinite ammo to be problematic. BL3, by and large, doesn’t balance weapon effectiveness around ammo efficiency, even if it pretends to.
So let’s be real, infinite grenades aren’t a balance problem in and of themselves, they only become a balance problem because the synergy between beam grenades and Vampyr allows infinitely spammed beam grenades to make you effectively invulnerable.
If that’s the root problem, surely the solution isn’t to prevent beam grenades from working with MoD and Vampyr, but rather to change the way Vampyr interacts with beam grenades to bring them in line with non-beam grenades. An internal CD, a limit to how many times a single grenade can proc Vampyr, scaling the heal amount based on the tick rate, etc. That would allow beam grenades to not only remain viable, but to remain a niche pick for ammo-limited builds, while not outclassing other types of grenades because of their healing potential. This would ideally be accompanied by some kind of buff to Vampyr that makes it worth using with non-beam grenades, because right now it’s usually not.