The plants are not weak, you just don't know how to play with them. The problem with this game is the sorry team balancing. No autobalance whatsoever and the players are free to switch teams wherever as long as there is a free slot on the opposite team.
End result, a zombie team (although I've seen it happening with plants too) ends loaded to the brim with super high end players pitted against a plants team loaded with low level players which inevitably end quitting, with due reason. And since game does not autobalance or anything, ends being a deserted plants team against a fully loaded and OP zombie team.
The truth is, I've grown bored of dealing with this and now whenever this incident happens I just quit the game, it gets boring as heck.