having something that strong % based at all would be gamebreaking regardless of any limits, and the lower the % , the MORE gamebreaking it is, because it is less reliable and more luck based
instead of a 20% chance to stun for 1 second with an internal cooldown of 2 seconds or something, it should be stuns for 1 second on attack, interal cooldown of 5 seconds, or something like stuns once every 5 attacks, counter resets after not attacking for 3 seconds
I think items with a % chance to proc should be changed from a flat % to a PPM (procs per minute) system. If you dont know what that means, it was basically the way quite a few effects from WoW were calculated to balance different attack speeds.
For example, crusader enchant (chance on attack to heal and gain bonus strength for 7 seconds) was a 2 PPM, meaning that regardless of attackspeed, if you were autoattacking the target for a minute, the enchant should proc twice. If your weapon speed (attack speed) was 3.8 seconds then you would hit the target less times each minute, so the chance per individual attack for it to proc would be higher. If you had an attack speed of 1.3 you would be attacking very fast, so the actual chance for it to proc per hit would be much lower, but over a period of time would equal out. Higher proc rates such as fiery enchant (chance on attack to hit for minor damage) were something like 8 PPM, which on a weapon with a slow attack speed was a much much higher chance to proc, (with a 3.8 speed weapon this became roughly 40% chance per hit to proc, but with a 1.3 speed weapon was around it was around 15% only).
What this could mean for LoL is that items such as phage, and any others in the future that would have a proc effect that was not on every attack would have equal chances to proc over a given timeframe of attacking, but champions with huge attackspeeds would not be garaunteed to perma proc, and champions with slow attackspeed would not be so random with procs