I've gotten tired of typing this over and over in multiple threads, so I thought I'd just type it once here.
SMGs are not nearly as powerful as many people think they are. There, bold statement made in bold type...go me.
It's understandable why so many feel they are the "hands down best weapons" in BO2. For starters, the smaller, more cluttered maps tend to create more short-range gunfights than we're used to, particularly now when a lot of players are still learning them. SMGs should win short-range gunfights against anything but a shotgun--that's what they're designed for. So if your AR or LMG is consistently being outgunned at short range, that's intentional and correct.
Now, the next problem: a lot of people don't grasp just what "short range" means. On U.S. Army M-16 qualification ranges, the closest target is 50 meters away. On pistol ranges, targets are between 7 and 39 meters away. Having qualified on both, I can tell you that if you miss a 50-meter pop-up target with an assault rifle, you're in the wrong line of work. Missing any target with an M9 pistol requires quite a bit of effort. "Short" range with a military-specs weapon is actually pretty long to most peoples' way of thinking...and that's not even getting into Marine Corps rilfe standards, which are considerably tougher. So a lot of the gunfights that you think are "mid-range" are actually short-range, and a lot of the "long distance" ones are actually mid.
Next, you have an issue which has existed in most every CoD title to date and which has been fixed in BO2: previous games in this series made other weapon types too powerful in close-quarters engagements. The result is that many players, particularly the really good ones who have tens of thousands of subscribers on You Tube (and start the in-game trends, whether intentionally or not), got used to using assault rifles and even LMGs like sub-machine guns. You can't do that anymore, or at least, you can't do that and expect to live when you run into someone who actually does have an SMG in their hands. The obvious example was the Famas in BO1: that assault rifle would outperform most SMGs from any range, even after it was nerfed. In BO2, if you're carrying an AR or LMG then you have to use them as they're intended in order to succeed: tactical movement (shoot & scoot) with an AR, and mostly fixed-position mid- to long-range engagements with an LMG. That doesn't mean camping...that just means that most of your shooting should be done from behind cover, and you shouldn't be barrelling across open areas or down hallways until you're sure that the path is clear.
Finally, we have lag. We all know it's a problem in BO2, and we all hope that it gets fixed as quickly as possible. Many of the other complaints about SMGs can be directly traced to this issue: you're not really getting one- or two-shotted by an SMG from 100 meters away, but on your end it feels like it because of the lag (compensation). If you go back and replay the game in theater mode from that other player's perspective, you'll see that he's getting more hit markers before you go down than you actually felt in-game (or that you were damaged by something/someone else at the time & that might not have registered on your end, either). One big advantage that SMGs offer is a much higher rate of fire than other weapons classes...so when the game is lagging in small amounts which you can't really see (but can feel), the high rate of fire on an SMG will often cancel it out at least partially.
If you are one of the people who enjoy watching CoD videos on You Tube, look up XBoxAhoy and watch some of his. He does very good, very in-depth videos on each weapon in a given CoD game which explains more about damage dropoff with range, recoil, and how a particular gun is best used. He only has one video up for BO2 so far (MTAR), but even watching his weapon guides for past Call of Duties will give you a better idea of how the guns are designed and how they work in the game.
As always, not intended to insult or belittle anyone or their complaints; only meant to help people better understand what's going on.