Some will contend that the perk system can unbalance a game. It is true that, if packed with the proper perks, a player can do some serious damage. But there are always counter-measures. When you are killed (and you will die quite often in multiplayer) you get a killcam view of where your enemy was located and how they ended your life.You also see the full list of their enabled perks. You can always copy them, or use other perks to try and counter this. If an enemy has Stopping Power, a perk that adds damage to bullets, you can pick Juggernaut for added health. It's in no way a perfect solution, but most who dominate online will do so based off their skill, not artificial means.
The only real balancing concern comes with weapon upgrades. It's going to be quite common that one person will reach level 30 using a wide range of weapons (and therefore not unlocking very many of the bonus scopes), while another will get to the same level using the same weapon in every match. That person will have an advantage, even if it is just a better scope. The only balance for this in game is that you can pick up the weapon off any enemy and use it as long as you live. That's nice, but it won't make a supped-up sniper rifle any less deadly.
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