well ap zac is basically the best asassin in this game once you get full build he is just killing any squishy in less than 1-2 secs.(not the best just joking But there is literally no way to escape him since he jumps on you with E then press W and ulti at the same time even if you somehow run away he will just Q and you are pretty much dead) What I am trying to say why they can become suddenly apc when they build ap. Ahri can not be tank when she build full tank items but a zac can become an ahri by building ap. You try to say anyone who can get ap hurt people its normal. Well, yeah but not anyone has that much high ap ratios. They are meant to be tank. I dont want to see an ap zac dealt 3x more dmg than our adc.
the problem is, this champs ( amumu sejuani zac) meant to be initator tanks, who can start fight and survive with dealing damage. This is why they have high ap ratios. They are not ap assasins. but since their kits are amazing, full ap sejuani can deal more damage than some ap asassins but what makes her op, her kit. She can just ulti QWE and the target is dead before he can react.
I know they are squishy since they build pure ap but that still doesnt change they will kill enemy adc then die. There is no raction time. People cried that much about rengar being op now rito will rework him. Why? Rengar can 1 shot enemy adc but after it , he does. Such as ap zac, ap sejuani.