I've become kind of a Sunflower main, cos I really like the character.
In my opinion, they have to be careful not to make each character a jack of all trades.
I think the issue is that not many people are trying to play this game as a team, for example, I've watched zombies run past a bunch of my teammates to get to me on the back lines and when I'm taking damage, nobody on my team is helping me. That's the real problem IMO, because I shouldn't expect to win a 1 Vs. 1 against an equally skilled player that's playing a DPS character as that's not my role, but when your team aren't watching your back you feel like it's every person for themselves and that's what can make it frustrating.
That's also what can make Sunflower's sunbeam ability seem like a death sentence for yourself, because you're basically a sitting duck as you're unable to move, but there are characters in the game that have shields who could protect you while you're using sunbeam, except they just don't. It's hard when there's nobody talking on the mics and the game is kinda hectic due to the high player count.
I would say that it's Scientist that needs changing, cos he's got very solid damage capabilities but his primary healing sucks. With a powered-up Sunflower you have 2 healing beams that heal a lot faster than Scientist's primary heals while also having a much better range, plus your secondary healing, while nowhere near as strong as Scientist's, provides AOE heals that are great when teammates are all grouped up and fighting to take/defend a point. Sunflower's additional overheal is ridiculously powerful too, especially when combined with the 2 healing beams.