Change Devourer, Rather Than Nerf It.
I do not think you should nerf it. I think you should change it. I'm fine with it being really powerful. But powerful on any champion that benefits from attack speed. be it Irelia, Vi, shen, sion even (good on his passive), champions who don't have on hit in their kits. Get to the root of the problem. And that is removing how its interaction with champion kits. In its current form it has too many variables to balance each champion according to it.
Posted this a while back:
http://boards.na.leagueoflegends.com/en/c/gameplay-balance/7ilJXEab-devourer-idea-burrowers-bounty This active allows to you release three auto attacks worth of on hit effects into one auto. But your next two autos will be devoid of on hit effect. (so your regular auto + borrows next two would-be-auto's on hit effects). This would be similar to hydra in that it will have low CDR on it maybe 5 seconds. and or whenever you payoff the borrowed autos.
You don't have more dmg over the long run, but you can compact your damage with this item.
if you want to make it scaling: maybe increase the dmg based on how many stacks you have up to a maximum. and maybe a bit per champion kills. that you can lose if you die like mejais.
Another/Or Additional solution: Bring back
Spirit of the Ancient Golem > > Stats: > +250 health > +20 Armor > +10% cooldown reduction > Unique Passive – Tenacity: The durations of stuns, slows, taunts, fears, silences, blinds, and immobilizes are reduced by 30%.
This way champions like volibear nocturne... can opt for 
instead of devourer. Thanks to the tenacity.
Ancient golem+
