Currently there is a 'soft' cap for the benefits due to the hard cap in place for defense which is 80%. As for the other benefit of each stat those currently increase indefinitely. So depending on your class/passives/gear you'll start getting less out of them somewhere in the 100-150 range.
To expand, there's two classes of benefits, basic survivability, and offense. Survival broken down to health which has no cap and defense, which caps at 80%. Offense into attack power (willpower) and critical chance (cunning) which caps at 100%
Note promotes are not the only way to increase critical chance, or to reduce damage.
Health
Got enough health, you're alive, don't have enough, and you're dead. However, with enough heal on kill, offense and defense combined, any additional health is less useful because you're already surviving. Or if you have barrier all the time, health is not useful. That said, more health allows you to be more lazy with your playstyle and there is no limit except time.
Defense
melee, ranged, magic. These are capped at 80%. Note magic resistence is seperate from magic defense.
There is no gain in defense benefit beyond certain plateaus, and less relative benefit even with absolute gain in stats indefinitely, depending on class/gear/passives/conditions.
Defense plateaus being different for classes example, a 1H warrior can gain Turn the Bolt passive, which raises their ranged defense by 50 and is additive with front defense of shields. From the front condition and 1H Warrior class, there is no additional defense benefit from cunning.
Some classes get cunning or consitution or willpower passives, and they each add to the promotes, and their respective defenses, lowering the overall requirement to maximize benefit.
Offense
Offense is broken down to added critical chance, which benefits critical damage bonus and passives, and attack power.
Attack power affects every attack ability differently on the damage formula from trivial benefit (1% to 10% gain in final calculated at most for a 100-200 willpower gain) in abilities with lots of 'damage bonus' on the ability itself (Power Chord, Longshot) to 1:1 gains in abiltiies that are pure weapon damage (Bolt)
Critical Chance: There is mathematical proof on geometrically diminishing relative benefit to critical chance. The damage component of critical chance however can be boosted with Critical Damage Bonus gear, maxing Critical Chance more valuable as CDB gear and dexterity passives (boosting critical damage) gets equipped.
You'll need more cunning to gain the same amount of relative benefit gained previously for any set amount. The benefit from going from 5% to 10% critical chance is a factor x2, and requires 10 cunning. To gain the same factor, you need to go from 10% to 20%, requiring 20 cunning. So on and so forth, the relative marginal benefit reduces.
Classes that have automatic critical chance passive benefit less from higher cunning as critical chance becomes guaranteed. Sneak attack and Opportunist benefit the most at about the 50-100 range. Mercy killing / Death Blow does not benefit at all.
Critical chance dependent passives sometimes have internal cool downs, such as Flashpoint's 10s cooldown. Once critical occurs every 10 seconds, additional critical chance is less useful. Also less useful once criticals occur frequently enough to overlap.