I am just going to go ahead and say......
Plants legendaries are mostly TERRIBLE. The only two that I can think of playing are the great zucchini in a super controlly citron, and the three headed chomper. The rest of them are super meh. I have a hard time wanting to include any plant legendary.
On the contrary, Zombies need the super expensive legends because they are more likely to stick around, or else plants will just combo w.e they want and they can remove everything, and the tricks become trivial because of that. It is much easier to build a board as a plant because of that.
I have a lot of success with just rare or below cards in plant decks, where as a zombie i have a really hard time (i basically have to tryhard every time), if I don't have legendaries.