Why Zac is bad right now.
This isn't saying the rework is bad, but rather explaining why his winrate is so low.
It's because Riot underestimated how strong old Zac ult was. It was one of the most versatile and useful ults in the game. You used it to chase, escape, as teamfight disruption, as burst, in-fight sustain, ect, ect.
So when Zac's ultimate got changed, his options as a champion were severely reduced, but all of that power budget didn't really manifest anywhere else. Sure his new Q is very fun and unique, but it isn't vastly stronger than than old Q in most realistic situations. In order to get Zac to a decent winrate, they are going to have to inject some of that lost power budget somewhere, whether it be damage, cooldowns, or more utility.
My first suggestion would be to revert the E knock up nerf. It would help Zac gank better and be able to pull of his new ultimate more.