Weird. In my eyes both of your example teams have both strong and weak champions. Their range is pretty much the same too and so is sustain. Both even have a wall.
To clarify I tell my view of the teams:
Team A: Strong ones are Darius, Sona and Teemo. Weak one is Yasuo. Master Yi is special case; if enemy has enough CC, he sucks. If not, he rules. Soraka, Karma and Braum have CC against him - and Lee can kick him away if he tries to come in with his ult. Would that be enough would be determined by skill of the players. They have moderate range as a team with 3 melee guys. Only Yasuo himself has knockup or do I miss something?
Team B: Strong ones are Karma and to some extent Soraka. Weak one is Lee Sin. A tank is usually a must have, so Braum is good thing to have, even though he is little weak side in Howling Abyss. Evelynn is little on the weak side, but can work if built to fix weaknesses of the team. She can function in many roles after all. They have moderate range (Karma Q/RQ and Soraka), and a better tank. AD Bruiser Evelynn to combo Braum stun on Master Yi. I would put my IPs on Team B, just because they are slightly underdog. :)
Edited: Team B has a tank. I am not going to call Braum better than Darius the Juggernaut now that I think of that. :)
Game balance is in the eye of the beholder, and game isn't over until nexus goes boom!
To the OP I say: Do not respond verbally or with in-game actions to the flamers. Mute and report them after match are the only response you need.