Considering Gram is dealing overwhelmingly slash damage as base I thought Buzz Kill’s +120% slash damage would be a slam dunk on it, yet none of the popular builds use that mod.
Does this have something to do with how damage is calculated that I’m missing or are physical damage increase mods just always a bad idea?
Without getting super into it, Physical mods are essentially a scam of a mod.
They do not affect your base damage, so they don’t also improve your elemental mods.
The effective damage increase is only a fraction based on the baseline ratio: using Buzz Saw on Gram Prime as an example, a +120% slash on a weapon with 75% slash results in a total +90% damage.
The real nail in the coffin is that the bleed status does not scale with slash damage, it only scales with base damage, base damage mods (like Pressure Point or Condition Overload), and double dips with faction mods (turning a normal +30% Bane into a 1.66x multiplier, and a primed Bane with +55% into a 2.4x multiplier), while also being affected by other damage multipliers (like crits, headshots, etc).
There are legitimate uses for physical mods, but for regular melee use, the opportunity cost of using them is high, when you could slot in other things to pump up your damage output higher. A regular Bane will almost reach the same damage output as Buzz Saw on Gram Prime (of course vs the specific enemy), while also increasing your Slash proc damage further.
Legitimate uses include bumping up a slash ratio to being above 50% for dismemberment (to produce more Desecrate targets); for certain weapons (eg Shaku is full Impact) and stat stick abilities (Landslide is full Impact) that only deal 100% physical damage to get an actually effective +120% damage buff; for certain stat stick abilities (eg Whipclaw) that don’t benefit from something better like banes, and can fish for more slash procs with it.