Anything that requires timing and smart play to be power is NOT by itself a powerful card. This is why bananasaurus and double mint are rather weak legendaries in my opinion. A lot of other cards can become unstoppable at right circumstances, and it is easier to do so.
Example: Pepper MD - ceiling? Couple times at turn 8, I made 2 10/10 pepper MDs, more than often I am able to drop this at 6/6, and if my opponent wants to use locust swarm on it? GREAT!!! Now I drop my super phat beets for 6/6 7/7 body. If I get the heals rolling early, and this thing grows since an early turn, even better, much more useful than double mint. For 2 mana that can often force your opponent to use 5 brains to deal with? I consider this card far better than both leg mentioned above.
Hibernating Beary - if your opponent is tapped out of brains, and you placed this at a 2 dmg or 3 dmg zombie, you can basically allow this plant to go off the charts at turn 4 (which they don't have access to locust swarm).
Blackeyed bean - I consider this the best card for plants, because many zombies are essentially forced to play tricks because of the turn order. If you put this bean at the right spot, it will force zombies to consider it twice before they use a trick to eliminate another plant. This is basically the insurance policy for plant heroes.
The reason why the great Zucchini is good because even if they eliminate the body, the damage is already done. At 9 mana, this is the best counter to someone who is playing giant Gargans like octo, sea monster, wizard etc. This also kills Octo's afterlife, which gives an insane value against a really good
I consider three headed chomper excellent because of the turn order. When played, zombies are essentially forced to use a trick on this. This is because you would place this plant at a place where it essentially protects 3 lanes and not get attacked. If they want to trade a locust swarm for it, fine, it is a pretty fair trade. If not, and this plant is able to eat 1 or 2 zombies, it already earned its value.
When you have to play it a certain way, or do certain thing in order to make a card powerful, then it is not a powerful card. The best cards are the ones when played (at any time) do one of the following:
1. immediately rewards you with value equal to or higher than the card's cost
2. Forces your opponent to immediately to spend equal or more resources to deal with. If they don't, it immediately rewards you enough that you gain a MASSIVE advantage.
PS: The trickster is the best card overall in the game, because it can get really cheap, and it deals damage immediately without any set up.
Also, I consider octo zombie a really good play, because of the afterlife. This means that this is 2x 8/8 frenzy. Which is really hard to deal with in the late game. This is especially true if you forced plants to use squashes on sea monster, or some other card already.