Why Zombies decks are so weak after Galactic Gardens?

I don't know why many players keep complaining about Zombie decks being too weak. I have all the heroes and like to switch between both sides to try new strategies - and I think both Plants and Zombies are pretty much equal.

I got to Ultimate League using 4 decks in rotation: Solar Flares, Night Cap, Impfinity and Prof. Brainstorm and I found they are all strong in their own ways. So I think the problem is your strategy, maybe you need to make some changes to have a better synergy between cards.

Good luck!