The most glaringly disadvantaged counterpart for the plants is the Sunflower imo. It feels so much worse than the scientist as the scientist has:
1. Better ability choices across the board. Tier 1 - Scientist has Heal Beam or Heal Station, Sunflower just has two Heal Beams that are exactly the same. Scientist is the clear winner because it has a self sustain/AoE heal option the Sunflower just doesn't get. Tier 2 - Scientist has Warp or Energy Warp, massively needed tools to keep a squishy 100 hp character alive, while Sunflower has the Sunbeams which are a very poor mans version of Gatling/BBB and basically a death sentence for an easily recognizable 100 hp (now immobile) character. Though both are different and sunbeam does have its uses, Warps are far superior here for both offensive play and for getting to where you need to be to heal/revive. Tier 3 - Scientist has Healing Grenade or Sticky Grenade. Sunflower has Dark Flower and the two Heal Pots that, once again, are essentially the same thing. This is the only tier I'd say is roughly equal - Dark Flower is the only interesting ability choice sunflowers get but is severely limiting by requiring you give up your only self heal, whereas the Scientist doesn't have to make such a sacrifice to take Sticky Grenade or Heal Beam.
2. Overall better weapons. The Astronaut has more DPS than every sunflower besides the Mystic flower spamming fast attacks. On top of that, Scientist has aggressive shotgun variants that play differently, whereas every Flower pretty much plays the same way with the notable exception of Vampire Flower. Vamp Flower is the only saving grace for the class IMO, but its limited health make it very punishing to play.
3. No gigantic head. As a sunflower your 100 HP is compounded by the fact that you stand out from the crowd very visibly and have a huge head for a target, causing you to take more hits overall, most of which will be crits to your low health pool. This isn't as important as #1 and #2 but it adds up to make the Sunflower a big steaming pile compared to the scientist.
I suggest bumping all Sunflowers besides Vampire and Metal up by 25 HP to counteract the fact that they have no mobility skill like the scientist. Metal Petal needs a substantial speed boost to only be slightly slower than the rest, and Vampire is fine the way it is (high risk, high reward).