So I'm not the highest elo, peaked p1 last season but I think karthus jgl is just game breaking right now. Not sure if this is such a problem in higher elos but In lower elos below mid diamond I don't think teams have the co-ordination to consistently shut down an afk farming late game beast. You take one bad trade and you are dead, now I'm willing to say this is fine as long as there is a way to punish a karthus however what happens is ok maybe he gets killed once or twice in the jgl if your lucky. But for the rest of the game he is just sitting there scaling waiting for cross map snipes getting fed with 0 effort. And then when the main objectives spawn he can press r, you look over at your half health support pinging everyone out and the adc scrambling to life steal off anything and that's how you lost baron/elder dragon. Who in their right mind would even think about picking a farming jungler like shyvana when karthus jgl exists in its current state, I mean its absurd.
Having said this lane is Karthus punishable, he lacks mobility and is squishy early game which are his weaknesses but when you can ignore all of that and still have the amount of impact he has there is something wrong.I will be banning karthus from now on until he is no longer viable in the jungle, I don't think lane karthus should be nerfed despite his kit in itself being toxic so something along the lines of just making it very inefficient for him to jungle would be ideal.
Correct me if I'm wrong, I don't claim to know this to be fact. Maybe I'm just bad and don't know how to play against this but this is just my opinion.