Sunflower definitely needs a nerf, NOT a hard nerf, but her healing other plants is a little too powerful. Makes other plants too tanky and difficult to kill in my opinion. Even when you catch a plant at low health, it can be difficult to finish them off with a Sunflower healing them. Healing per second shouldn't outmatch Damage per second by too much, but just enough that a Sunflower and a Plant will likely win the matchup against 1 Zombie.
Survivability: Both characters excel a little too much in survivability. A Sunflower that knows what they are doing and are using right upgrades can be difficult to kill, especially since they'll be with a teammate, as they should be. I think the major problem with Sunflower is Healing Hand's healing output.
Scientist having 3 Warps, that have No Startup before Warping, top that with a 9 sec cooldown and 3 charges it is very spammable and not very difficult to get out of harms way. I think Warp should have a Start-up before activating.
Scientists warps also essentially make him a decent character for finishing off plants, and he has good damage, although I don't think Scientist being able to pick off plants is a problem, just the current Warp might be making this too strong.
I have killed plenty of Sunflowers and Scientists to believe that their survivability is not strong enough to help them survive alone. However, Healers shouldn't be by themselves anyway, and most of the time that I do kill Healers, it is because they were out of Position.
To play a healer you need good Position and Game Sense. You need to know when to heal, and when its time to leave. You got Sniped? Bad positioning. You got killed by a Drone? Should have been looked out for drones. Enemy flanks and targets you? You should have checked your flanks. Got killed even though you were near teammates? In this case, that's your teams fault for either not killing the enemy, or shooting a drone, but most of the time a Healer gets killed because they weren't paying attention and got caught. Keep your distance from the enemy, stay near teammates, and know when to get out of the way and when to heal.
OverHealth: My only problem with Overhealth is that it's permanent. Overhealth really important for keeping your team alive, and is something that you should take advantage of. But Overhealth should slowly degrade overtime, because being able to give permanent Health Buffs to your teammates is extremely powerful for a Healer, which means that if one team is using a Healer and the other team isn't, the team without the Healer is at a huge disadvantage, especially in 8v8.
Speaking of healing, I'm going to talk about Defibrilated for Outta Fight (Electric Slide) and Epiphany for Arcane Enigma (Rose). These upgrades should not work the way they do. Electric Slide and Rose already have great survivability, so being able to Heal on top of Reloading while being Invincible is ridiculously overpowered. There is also no start up to Outta Fight or Arcane Enigma. With these upgrades, you could theoretically turn a losing match-up into a Winning match-up by pressing a button and waiting a few seconds. I'm fine that Defibrilated doesn't work right now, because that and Epiphany are terrible.