The problem is both Cursed Blade and Gunslingers themselves.
Gunslingers by nature are THE best on-hit users by a fucking mile since they're the only ones who apply them in an AoE bar Volibear and Runaans (building Runaans tho omegalul).
This means all on-hit items inherently are going to be balanced around Gunslingers, meaning they'll always be subpar on everyone else. This is the same shit that we had with WotA being a Vladimir item since he used it 10x better than everyone else so they had to nerf it so much that it was basically deleted for everyone other than Vlad.
Imo Gunslingers' secondary shots just shouldn't be proccing on-hits. They should just be the multi-target physical damage origin. Buff their synergy ofc but just remove on-hit functionality because it's going to inevitably end up killing Hush/Disarm/Cursed Blade for every other class/origin if it hasn't already.
And then Cursed Blade itself just shouldn't exist period. Reducing by a star level essentially cuts a unit's AD and health in half on top of a massive reduction in their spell's power/effects, all on a 20% chance proc. It's even more ridiculous that it can do this repeatedly to an already reduced unit.