i think the biggest problem with plants is they have no card draw which makes it harder especially for better players as they generally go for a more controlling strategy that involves card advantage. although plants have sun acceleration but that is just not good enough as it is very easy to lose it and you start with too few cards to make a difference.
they should let sun flowers for instance let you draw an extra card + an extra sun so then it would make sense to play them and defend them. or at least make the sunflower give you a permanent +1 sun(like the signature spell) and just be an extra team up body. and also introduce more spells/creatures that draw you cards + add a little bit of card draw to some of the buff spells.
as it stands now most of my games with plants come down to top decking or just killing them quickly enough so it doesn't matter but the zombies always have more cards than me at the end. i mean they have so much draw in different classes - img, the 2 brain 1/1 dude that draws a card , kite flier, teleport, the move spell (forgot name)