If Call of Duty Black Ops were actually balanced, half of the people I run into wouldn't be using a FAMAS. An AK-47u with Rapid Fire is another common one. Anything with a fast rate of fire will win out every time, and the only way to stand a chance (assuming the players are competent) is to use the same thing.
They patched the FAMAS to give it slow aiming time and poorer hip fire accuracy than other guns. Its also weaker and takes more bullets to kill, although with the high rate of fire that's not much of an issue. Alot of players consider it a Noobish weapon, but its not necessarily superior to other guns.
I think the AK74U is superior to all the other SMGs, though. So I do agree with you on that. It has the same damage and rate of fire as the MP5K but unlike the MP5K it has the Grip, dual mag, and even grenade launcher attachments. I also think it has a bit less recoil and better reload times also. The only thing the MP5K has going for it is its available immediately, but once you unlock the AK74U there's probably not much reason to use the MP5K ever again. So I agree its not very well balanced in that respect.
But when I said Black Ops is balanced I was mainly referring to the perks and stuff like that. Some guns might be a tiny bit better than others, but its not severely unbalanced. A player with an MP5K isn't at a huge disadvantage against a player with an AK74u, even though the latter gun has more attachment options available. Both guns do the same damage and similar in almost all respects.
And Rapid Fire is not necessarily the best attachment choice. It increases rate of fire, but it comes at the cost of increasing recoil and you will deplete your magazine much quicker and need to reload more often. It makes it more powerful at close range, but you will find at medium or long range it makes it harder to keep your shots on target. So you do get some advantage, but it comes at a cost. So I think it is fairly balanced. You can also put rapid fire on the MP5K and any other SMG if you want to, so its not exclusive to the AK74u.
Maybe you are playing the Wii version of the game and maybe that hasn't been patched... I dunno, but on the other systems Treyarch did nerf those guns somewhat, so they aren't as unbalanced as they once were.