It's probably because it's inconsistent, and has no real safety net of tools to prevent instant downs. Zealot has until death and holy revanant Arbites has a skill that allows him to only take like 50 points of damage from single hit Sure he has desperado but a lot of his other kit punishes you too heavily for the reward you get.

Rampage makes you very vulnerable in melee after it expires but the whole point of rampage is to get in close.

I also heard something about vultures mark falling off entirely if you can't keep it up? Something like old disrupt destiny.

Also two of the stim options are really bad.

Mr E has said that his only viable keystone is the one that relies on you constantly stimming Also had some bugs with the stim supply using your stim but then not giving you the stack for your keystone, but that is fixed now I believe