The issue with devourer is just that huge power spike once you get it stacked.
That phantom attack while fun in concept is the overall problem with the item in general since it is a huge power spike for champs who can make good use of on hit builds. Yes this includes Irelia and Master Yi who both have non damage over time on hit damage in their kit.
Might jsut be me but the overall idea of making it a little better early to mid game and scale reasonable well but not be all about maxing out seems like an overall better way to go about the item.
Perhaps making the item be something like
35% attack speed
35 on hit magic damage
Killing big monsters grants 1 stack, kills/assists 2 stacks, dragon/baron 5 stacks (only if your team gets the kill), 4 for an assist or kill on a tower; max of 50 stacks; every 2 stacks proves 1 more on hit magic damage and 1% attack speed (max of 25 from these stacks).
Make it less ideal to focus just on farming the jungle since it would not be a simple task to get max stacks by 25 minutes in this kind of situation and there would be no massive power spike for getting it maxed regardless. Instead it would be a gradual power gain over time for doing things a jungler should be doing. Aka ganking, clearing some camps, objective control and overall map pressuring.
Sated Devourer should be a limit not the means of making it a worthwhile item. The power should be scaling within reason over time instead of jsut bieng all about maxing so you can kill everything.