"Mechanical Acuity: This item now grants a stack of Mechanical Acuity for 4 seconds whenever you deal non-Critical Damage, granting you 20% Critical Chance per stack, up to once every half second. After this effect ends or reaches 5 stacks, it cannot occur again for 25 seconds."
This is more or less a "pity mechanic." You are more likely to crit if you haven't done critical dmg recently. That idea in of itself is good, but the way it's introduced here is bad for two reasons:
1. The ramp is TOO RAPID. Having each stack give 20% crit is still a lot. Each individual stack is full of a lot of power, and as such the set needs a long cooldown to balance out that power
2. The cooldown is TOO LONG. Who wants to wait 25 seconds to be able to use their set again? The set that ensures that they start to rapidly build up burst? During that downtime there is a substantial decrees in power
Here is my solution:
1. Change the way it stacks. For starters, greatly reduce the amount of crit chance each stack gives. Then increase the number of stacks. Lets say, whenever you deal non-critical dmg, you get a stack of acuity. A stack of acuity gives you...4% crit chance. This effect can stack up to 25 times. With this variation, because the ramp up is much slower for a lot of builds that aren't pumping out lots of constant dps the stacks should last infinitely as long as you are in combat
2. Now then, because the set slowly ramps up more, I feel the way it should "unramp" should be changed with this variation. Once you deal a critical hit/critically heal you should lose ALL your stacks. Then there should be either no cooldown for when you should be able to gain stacks again, or a very short window of maybe 3-5 seconds
The wording would be something like this: "This item now grants a stack of acuity whenever you deal non-critical damage, granting you 4% critical chance per stack. This effect can stack up to 25 times. Once you deal a critical strike you lose all your stacks"
Now there could be some cooldowns somewhere in there. Maybe keep the half second cooldown per stack in order to ensure it can ramp efficiently and you don't just gain like 15% crit chance after a small series of moves (such as aoe dots being able to hit multiple things) don't crit. That would be reasonable. And then the before-mention introduction of a small cooldown on the stacks after losing them. I'd rather not have the last one, because what if you miraculously crit once you gain a single stack because the RNG gods blessed you? You could also add critical healing to the set effect, that way the defensive power of the set is kept in check