Is the Human Element. Allow me to explain (this might end up being a long thread just like everything else I write).
There are two elements to the game, AI and Human. AI involves things that are outside of a players control or heavily influence a players actions. This includes gun damage, RoF for full-auto, spawns, and obviously AI controlled Pointstreaks like Riley.
Then there's the Human Element; parts of the game that are based on the human capabilities. This includes things like decision making, accuracy, RoF for semi-auto weapons. All these are dynamic elements that vary based on who the player is and how they function.
The reason I say the Human Element is OP is because a player, such as me using the Mk14 from MW3, knows how to best use a weapon. However, in the same breath, I could be given the PP90 and not be damn bit useful. Someone else could be the opposite. Incompetent with a semi-auto but insanely productive with fast firing SMG.
If you actually take the time to analyze everything through this concept you'll realize just how hard it is to balance parts of the game. There are hundreds of different factors that determine the rate of success one might have using a feature of the game. You have enviornment, opponent tendencies, teammate tendencies, gamemode, map, ect. All of these factors and more combine to help sway results. Ultimately, though, it's the Human Element that determines how effective a feature is.
Let's break this down further using my Mk14 reference above. I'm a Socratic thinker so I like to obnoxious about points.
Player A (me):
When it comes to combat I'm not the best with reflexes. I operate on 7 sensitivity only after several Monsters. I do not like to over extend myself so I stay in areas with long lines of sight but I do like fast tempo so I stay near high traffic areas. I do have a fast trigger finger and great rhythm so I can fire a semi-auto consistently at it's fastest (or near fastest) RoF. I'm also good at using sway and recoil to my advantage. Knowing how the weapon works allows me (myself) to make-up for it's weaknesses. This means I'm the ideal user of a semi-auto.
Player B (not me):
Plays fast and loose using reaction time to it's advantage. Operates on the highest sensitivity and even mods their controller to make it faster. Knowingly moves faster than they can think so they stick to tight spaces that to try and contain their field to only what's inside their crosshairs. They don't bother with taking time to aim as they can't afford to do so. They put themselves in the thick of things as much as possible. Ideally this person wants an SMG that shoots fast, does not hinder movement speed, and has a large magazine.
These two players, in their respective preference, have needs that are met by Call of Duty in a balanced manner. The weapons, Mk14 and PP90 (we can even say FAL and Skorpion) fit their most preferred scenario effectively.
However, if you put Player B in Player A's scenario what will happen? Obviously the Mk14 will win and if the opposite were to occur and Player A was placed in B's scenario then player A will fall victim to Player B's advantage.
Now let's ponder a third and fourth scenario. What if I'm Godly accurate and have a trigger finger that is uncomprehendable . My timing is like clockwork and I move so fluidly and precisely that even Player B with all his advantages cannot defeat me. In other words I'm better than him. No matter what gets thrown at me I cannot be stopped with my Mk14.
So now I have ability to win in my own scenario and in Player B's scenario. From that outside looking in what does that look like? Well it obviously can't be me because Player B is clearly in his comfort zone and I'm not so it has to be the gun. There's just no other explanation. The Mk14 with a third of the RoF and only one less shot-to-kill is overpowered, right?
It's this kind of thinking that creates unnecessary mob mentalities. Players who don't think about intended purposes and the fact that a weapons intended purpose does not mean it's exclusively for that purpose. A player can do whatever they want to do. Sometimes it might not be in their best interest but if they work at it hard enough and the players they're going up against cannot successfully defend against it just might work.