The three Scientists shown n your screenshot are the Marine Biologist and the Chemist; both are widely acknowledged to be the most dangerous Scientists at close range an if a crafty and skilled player can get them into the middle of a group of Plant players who didn't see them coming it can easily result in a massacre.
It can be difficult, especially with Scientist players who know what they are doing, and nobody is denying that but the Scientists don't always have the upper hand. Case in point just this morning I was working on getting two Rose variants up to Master, the default and the Toxic variety, on Turf Takeover. The Zombie team featured at least three or four Scientists and while they did well over the three or four games I played I was able to vanquish most of the group more often then they got me- many of our shootouts occurred while they were actively heading in my direction as Rose is generally an easy vanquish if a Scientist can get in close but a lot were me catching them off guard from a distance.
In the games where I know that a Scientist is flanking around behind our lines looking for easy ambushes I personally try to counter that by hanging back behind the main team to see if I can catch which route it being used and generally speaking if you can catch them in the act of coming around the flanks a few times, either by shooting them at a distance, flanking them, or coming up behind to ambush the ambusher you can sometimes get them to back off on the aggressive play; either that or have them specifically start coming after you.
As far as Sunflowers being on the Boasting Pedestals at the end of the game, I've been there many times with various Sunflowers not only with Revive or Healing Boasts but also with the largest Streak or Most Vanquishes as well. Once you level up your Sunflowers to get the reload, ammo and damage upgrades if you have good aim, decent tactics and engage the enemy at the proper range it's not that difficult to be one of the top performers overall in terms of vanquish count or overall score.
At the end of the day the it's my belief that the characters are pretty decently balanced; the factor which can turn almost any character into a game breaking monster is the individual level of experience and skill the person playing that character has available and whether or not the opposing team either has someone who can serve as an effective counter to that sort of player or can group together effectively to neutralize them.
