Well, really, this only applies to the ACR and the MP7, but let's get going anyway.
Why are these weapons so frowned upon by the community?
I mean, the only thing wrong with them are the recoil patterns and the precision they provide the users. But neither the ACR or MP7 are outstanding in any other category.
The ACR is the third slowest firing fully automatic AR in the game, and the iron sights aren't the most fascinating. It also doesn't exactly have the range that the M16, MK14, or the AK47 boast, so its damage from that impressive 45 initial damage drops pretty steeply. The main draw of the ACR is really, just the low recoil. It still bounces in random directions, but it's very managable nonetheless.
On the other hand, the MP7 has the lowest initial damage of all the SMGs, tied with the PM-9, and its range is terrible, but less steep than the P90 and PP90M1. It has a slightly bigger mag than usual, but it's not the biggest. The reload time isn't all that fast, either. Rate of fire is decent, but still slower than the PM-9 and PP90M1, and of course, it won't matter against a good shotgun user. It also would make for a terrible long-range option if it wasn't for its extremely low recoil. It's so low, it doesn't even bonuce in random directions, but rather rises up very slowly, and that can be compensated for easily.
But... that's it about these weapons. I don't see anything that stands out other than the recoil. Even then, it's all about the player's accuracy.
Because with a weapon that doesn't kick as much, a terrible player would find themselves unable to use these weapons effectively, finding their ideal weapon elsewhere. This is because these weapons are precise, but depending on the circumstances and the skill of the player, it can be made totally inaccurate. You can give a really bad player a PM-9 and an MP7 and tell him to shoot at multiple targets, only to find that he did better with the PM-9 because he was such a poor shot that the kick of the PM-9 helped him hit more targets, while with the MP7, he mostly shot inbetween the targets, but the spots where he placed a group of shots are unusually close together. You could probably do the same with a CM901 and an ACR to find similar results as well.
What I'm trying to say is, they're really only considered "OP" in the hands of good players. Sometimes I see people with MP7s and ACRs doing terribly while I'm using something like the UMP45 or the AK47. In fact, I found my KDR rising as I was using weapons with slightly more recoil, because I know my aim is inconsistent, so much so that I occasionally miss my shots entirely with the ACR and the MP7. Until now, they've been my favourite weapons to use and the ones I've gotten the most kills with. But I often find myself using different weapons and doing even better with those. Nowadays, they're only backup weapons, and probably weapons I shouldn't have used my Prestige Tokens on. But what can you do when you like so many weapons and can only bring a few of them along with you when you Prestige?
And yes, some may argue that my example is overexaggerated and that everyone has decent accuracy, that being augmented by the precision these two weapons give you. But that's the only reason they're used more. They're easier to use.