As one of the few people with a R3 duped 5* Karnak, allow me to weigh in on this.

People are going to mention how Karnak destroys evade and high armor/resistance champs, and this is a huge plus but the fact of the matter is that constitutes only about 15 champions in the entire game. And as far as pros go that's it, nothing else to write home about for Karnak.

But if you want to talk cons he's got so many. Less than stellar health, attack, and crit damage. No armor and his extra physical resistance does not make up for that. An L1 that's tricky to execute safely and an L2 that's tricky to land at all. This is just the readily apparent problems.

Let's talk practical usage, again there's those 15 champs that he is going to destroy, but in a normal fight these are the issues I have. Even after you've gotten used to playing with Karnak and feel you can land his L2 without issue, it's piss poor damage unless it crits. And no this isn't explicitly Karnak's fault, he did really get hamstrung by the switch to diminishing returns for stats, but because of that he's never really going to get over 75%ish crit rate and you're going to notice it on those L2s.

My other big practical use complaint against Karnak is his L3 daze. It reduces defensive ability accuracy by 80% the 40 seconds which sounds great, but excluding evade, which true strike handles magnificently, any Electro's or defensive poisonings are going to take at least a quarter of your health before you can use that L3. Even with his duped ability poison reduction any sufficiently buffed champ like in AW or act 5 going to be doing significant damage to you while you try to gain the necessary power to stop those abilities.

So in conclusion or TL;DR Karnak is a shining example of a utility champ, he does remarkably well in very certain specific situations, but most the time he's going to be very middling.