Disclaimer: THIS IS 100% OPINION
From a veteran CCG perspective, plants are A LOT stronger than zombies right now. The lower tier is still filled with players obviously new to CCG, so I can't even tell, but so far i am 8-0 as plants vs people ranked in league 30+ (i am still in silver, just hit rank 12, doing too many hero quests in campaign mode). I am around 50% vs rank 30+ as zombies, because it is really hard to respond as zombies.
Here are some common plant mistakes I see when I play as zombie in lower tiers:
1. Plant players fail to take advantage of is the fact that zombie players can't combo their creatures with tricks, so anything without a gravestone should die.
2. Don't play anything with 2 attack. If i am a zombie player, and the plant player just plotted down a 2 attack? I would be ecstatic, I let you have free swing at me as much as you want, especially if I am THE SMASH. PLEASE CHARGE MY METER. The super powers are ridiculously OP, I want to collect as many as possible in a game.
3. Playing a naked 1 sun for 1/1. Just trust me, don't do it, just don't.
4. Trying to overload the board as much as possible, overextending like crazy. Taking damage early will actually help. The only Aggro plant hero, Nightcap, is SOOOOO BAD, because you just charge up block meter.
My opinion on weed spray:
In the way this game is build, it is a SUPER techie card. analyzing the plant decks, there is no point in plants playing a lot of low attackers, if they do, it is used to trade for zombies to minimize early damage while still charge up block meter. Too situational, I as a zombie would only 1x or 2x, unless I know 100% that my next opponent is nightcap.
My power tier?
Chompzilla - WHOA!!!!! You get this guy for free? Why would anyone ever play zombie tier.
1. Early game removal AND late game removal? WHAT!!?? (mallet, squash, lawnmower, just take your pick)
2. Heal, so you can get all the super power you want
3. The most card draw in the game (sage sage, AND Flourish)
4. has access to some of the biggest creature in the game (I mean one time I dropped a 16/16 pepper MD, it got killed, and then I made a 10/10 pepper MD 2 turns later).
This guy definitely needs to be toned down a little, i think I lost 1 game so far with him, and that was with literally the worst possible draws (like bottom 1% worst draw).
Since Hearthstone is the prolly most popular CCG right now, I will use that as an analogy (doubt a lot of ppl play stuff like magic, shadowverse, duelyst, hex etc.). Chompzilla has removal like a priest (power word pain + powerword death), drops big creatures like a druid (also ramps like one), and draws card like zoo.