Indeed the game has been out for about 7 months so there are plenty of players here with lots of experience.
In my personal experience find that most people go through this series of stages:
First, while they are starting out, they think that zombies are overpowered and that the blocking system is gimmicky and makes the game too reliant on luck.
Then they start to play zombies and realize that plants have some very powerful strategies that depend on some more uncommon cards. They also realize that the shield/block system is actually quite interesting to use strategically and they start to do things like letting weaker opposing creatures attack unblocked so that they fill their block meter and use effects that whittle down attack so that the enemy's lanes are full of weak creatures just filling your block meter and getting in the way of their own strategy.
Finally they realize that plants have some rather overpowered legendaries (as they start to get them and more heroes from their accumulated gems from climbing up the ladder and doing quests) and start playing more plants again and then come to the forums to tell those people in the early stages that, fear not, it's actually plants that are ever so slightly better.
To those that refuse to believe the players in here saying that the game is not in fact imbalanced and that zombies are not actually stronger than plants, just ask yourself why would those people that have been playing for a while insist in saying that it's just not true. We don't have anything to gain by collectively spreading misinformation. As I said before, the easiest way to discover this is to start playing zombies until you reach that level of ranked play where you are facing plants opponents who know how to play them properly and then you will see the true power of plants and how they deal with weed spray, the chickening, gravestones, deadly, arm wrestler and all the other cards that at first seem impossibly overpowered.
(EDIT) OMG, So close to leveling up in these forums!