Kog'maw's rework made his kit dull and boring to use
Kog'maw's least engaging ability is his W, and now his kit is completely centered around it. In fact once you have enough attack speed, you can't even orb walk without losing DPS. There comes a point where his gameplay literally involves finding a good spot in a fight, activating W and just standing in place auto attacking. His abilities also only do 55% when his W is active, just in case you consider doing anything other than right clicking.
The increase in time it takes for his ult to reset back to its original mana cost has completely ruined AP Kog and really just the ability as a whole. His ult is honestly weaker in its current state than many non ultimate abilities. It's important to remember that his ultimate lost its bonus damage to champions, which means that his ultimate does MUCH less damage unless they are below 25% health where it only does slightly more. Also, don't even think about using it when W is active otherwise it will do even less.
This rework was garbage.
"Kog'maw, why didn't you run away from the Darius?!"
"Sorry man, I pressed W."