Not entirely true.
Season 1 zombies were more powerful than plants.
Season 2 has given plants a passive buff. Now they're stronger.
You can see this in how many players play each side. In season 1, it would be extremely easy to find a match if you played plants. There were more zombie players. In season 2, this switched. Now there are more plant players.
Being a plant main, I'm not switching sides. I'll just ride out the ebb and flow. I'm sure the developers are keeping track of the data for the next patch on what gets hit w/ the nerf hammer.