The problem is the allocation of all the kill skills he already has. Fl4k has the Bounty Hunter COM which is exactly like Seein Dead, only it procs Hunter skills as well as kill skills, the only reason the Bounty Hunter isn’t considered OP or central to Fl4k being a viable character is because most of Fl4k’s damage comes from their other skills increasing it passively.
Meanwhile the vast bulk of Zane’s damage relies ENTIRELY on kill skills, he has next to no passive damage boosts aside from Violent Momentum and Synchronicity. Even then, the only major damage contributors are Donnybrook and Playing Dirty if you spec it to get a 100% activation chance. Everything else is secondary effectiveness like fire rate and reload speed.
If Zane had his damage potential linked to more passive bonuses in his skill trees, the Seein Dead COM wouldn’t be as necessary to make him viable for harder difficulties. His kill skills could still significantly contribute to his damage potential, but they wouldn’t be the backbone of it the way they are right now.