Flack Jacket is a perk that alters character damage statistics in that it allows you to take less damage from explosives. Yet there is no perk in the game that increases explosive damage to counter it.
Bullets are not a counter to Flack Jacket as much as bullets aren't a counter to any other perk or attachment in the game. Everyone has access to bullets and bullets affect a player the same with or without Falck Jacket.
It is like having Juggernaut (damage reduction) in the game but no Stopping Power (damage increase) to counter it out.
When a game has one option to increase damage resistance but no option to bypass that resistance then it is an unbalanced game.
The only way Flak Jacket would be balanced is if Fireworks or Danger Close made a return.