I only played it for like 2 games, and I don’t think this is a keystone for me, but I think the problem is that it locks you out, or more precisely punishes you for doing something and that feels bad. I’m not sure how well I can describe it, so I try to compare it to other stuff.
Ogryn Heavy Attack Keystone makes you do heavies or heavy-light combos. It doesn’t feel like it punishes you for doing light if you mix in heavies every now and then and also has a node that makes lights keep up the stacks. It’s a reward for what you are doing with no punishing method.
Zealot’s Crit Keystone wants things to die, it pushes you for more aggressive play, but once it ramps up it’s super easy to keep up and is static. You even have stacks on crit to make it easier. You don’t even have to get the kills yourself. It has a super large area and on Auric its inconsequential to keep up working as a passive buff.
Zealot’s Mobility Keystone wants you to move and dodge, this thing comes super natural to melee playstyle, and while you get more out of it with a knife you can easily run it with a hammer. You could say that it punishes you for standing still, but in actuality, you not really standing still ever. Also has a node to work on dodges.
In comparison standing still when shooting doesn’t come natural you always trying to get a better angle. Letting you move when you get a weak spot hit doesn’t feel like a reward it feels like a punishment to not get one.
I wouldn’t argue that it’s hard to maintain or anything, it just feels restrictive in comparison to anything else. I have a pretty similar problem with agile combatant.
I think what could work if the stack would just fall off naturally weak spot hits would reset the timer on the stacks and the headshot would give you a stack with very lenient time (like 8s duration or something), and no stack lost on movement just on time. This would make it good even for a more mobile build (revolver or something) which I don’t think is a problem, but FS definitely wanted to avoid.