I think the champ by design just has too many things going for him. A huge heal and a decent shield, a dash, a lunge, an execute, a slightly lower range amumu q that not only stuns the target during the pull but knocks up, a ranged slow(with the range of the average adc auto), and an aoe cleave on his autos. Then when he steals ults the ad ratios get converted to ap for him, and he gets to cast them for free(stolen kass ult definitely isn't broken). I don't think it's that any one thing on him is overpowered, it's just that all of it together is. I think, if people are going to play him as an assassin, he shouldn't have the heal or the shield, or both. Maybe the ratios on stolen ults also shouldn't be converted.