The proc condition for this set was recently changed from 15% chance on any damage to 15% on direct damage. I think that narrowing the method to activate this powerful effect could have interesting results, but keeping it 15% with is too low. It should be increased to 25% on direct damage, which would still result in lower uptime than on live, but it could still be effective for builds that optimize around the proc.
Using magblade for an example, since they are the class that has been using Acuity most effectively. The difference between a direct damage tick and a DoT tick is massive. A single cast of path, wall, or cripple has 9-12 chances to proc acuity on live. No nightblade skills have more than 1 chance to proc the set with the direct damage change.
In a typical nightblade "rotation", 3s are spent on back bar refreshing DoTs, then 5s on front bar. On live, the back bar DoTs proc acuity reliably within 1s of going to front bar, which means the proc is active at least 4/5s of the highest DPs part of the rotation. On PTS, after swapping to front bar, it takes on average about 3.5s to proc acuity (2 direct damage per second, light attack and spammable, enchants are not counting for acuity). This means that the front bar rotation is nearly over before Acuity starts offering any benefit. Sure there are times that it goes off sooner, within the first 1-2s. But there are also times it procs just as you are leaving front bar to go refresh DoTs again, and is largely wasted.
With my proposed change to 25%, acuity would proc on average 2s after swapping to front bar, allowing 3s of benefit on front bar rotation. It is lower than the proc on cooldown seen on live, but higher than the average 3.5s delay on PTS.
The other way to try to force an acuity proc would be to run force pulse. This mostly applies to sorcerers, since going to a destruction staff spammable removes a lot of the utility of a nightblade. In this case, acuity will likely proc within 1-2 casts of force pulse, since it counts as 3 direct damage ticks. The trade off here, is that you are giving up the higher base damage and free damage every 5 casts of imbue weapon.
I think this reduction would still be enough to make other sets more desirable than Acuity for a lot of builds (particularly those with high crit, like dual wield daggers or Khajiit), without completely removing it as an option for most classes.