Kog was pretty crazy for quite a while

The Tank update hit Kog with a whole broadside of indirect nerfs, not only to items, but also to base magic resist. A major contributor to his jump in power about the time Riot buffed his W was the removal of the Legion MR aura. With MR up, the effectiveness of his OnHit damage goes down, and so does his win rate. It tanked by about 3%.

It nicely illustrates Kog's knife-edge balance. Factors that might seem relatively minor can have a pretty big impact on how well he does his job. Makes sense, due to the fact that his job is 'does ALL the damage'. There's a tiny sweet-spot where that damage is high enough to make up for his glaring weaknesses but not so high that it completely over-shadows them.

Kog could really use some work. Maybe not a VGU, but a stand-alone update. If Riot were to focus him more heavily on his ability to shred resist for his team rather than on his stat-check button, I feel he would have a much better defined strategic niche to offer.