This comes up in every single one of these threads, so we gotta correct this right away.

Zac is HORRIBLE in the lategame. One of the worst champs in the entire game. He literally has a 30% winrate in really long games, which is ridiculously low.

He scales for a while and is crazy strong when everybody is around 4 items. After that he falls off incredibly steep. When everybody is full build he sucks so hard, it's not even funny. Nobody will get hit by his spells with lategame movement speeds and lategame cooldowns and since he scales with health he dies faster than most bruisers as a tank. Also his damage at 6 items is so low, that every champ with the tiniest bit of omnivamp or lifesteal will heal on him.

It feels like he scales good because games are short, but do not actually expect to carry a true lategame on Zac!