current kassadin is not op at all. in fact I'd say hes pretty weak as he has been gutted beyond belief.
but thats because he pretty much has to be gutted sadly. kassadin's ultimate is just too powerful to give him power anywhere else which leaves him lackluster. Even faker and pawn got rekt at worlds this year on him. (was surprised they even picked him)
The ability to blink multiple times every second or so is just way too good and it makes him obnoxious when he is viable. never mind that he is also an anti mage as well which makes him the perfect pick into immobile mages as their isnt anything they can do against him once he hits 6. (think annie vs kassadin)
right now, if you pick him, you are picking him because you enjoy him imo. He is not strong or op. but his kit is cancer and toxic.
he needs a rework at this point. they have tried over and over to nerf him and it just leaves him gutted and useless.