My Problem With Devourer
When it's the only item you need to buy.
I mean, let's put it like this. Many people complain that Master Yi is a clear abuse case with this item, and I'm not going to argue that Master Yi isn't powerful with this item, but consider that Yi will often build entirely damage to suit this item. The only reason why he's tearing through Tanks and Squishies alike is because he often devotes all 6 of his items literally towards killing others as fast as possible. Virtually any CC that shuts down Alpha Strike or well timed counter engage will result in his death very quickly, and otherwise cancel out the strength of his build (except in maybe low level yolo Q, but let's be frank, Yi has ALWAYS been strong in low level solo Q, so making any argument about that based on Devourer is just plain wrong).
But then there are other Devourer users. Shyvana, Udyr, think of any tanky Raid Boss that likes to buy Devourer and really think of this for a moment. Yi will get a lot of damage out of Devourer, yes, but he comes with the rather extreme weakness for a melee of being incredibly squishy. On the other hand, Shyvana or Udyr will often buy Devourer, one other damage item maybe, then use the rest of their cash and item slots for Tank/Bruiser items to make them impossibly hard to stop.
This is really my problem with Devourer. I don't mind it when full carries invest their entire budget onto Devourer because then they often come with the weakness of being super squishy and often being melee. While you might argue that they're overtuned, you must admit the weakness and therefore balancing lever exists. What really irks me however is when champions with good synergy and tankiness abuse Devourer, as it doesn't come with the same weaknesses full carries often deal with, and in my eyes is really what breaks the concept Devourer; creating a late game DPS monster.
Honorary mention of Blade of the Ruined King in this post. BotRK has a similar problem in that its Life Steal and % per Auto Passive and the lifestyle that applies on that passive are all abused by Devourer, therefore giving full Carry Devourers often times a lot of bulk oftentimes to the level of a Bloodthirster while having superior DPS. Personally, I always thought that Blade of the Ruined King is the problem, but I just wanted to mention this somewhere in the post.