It's not a problem to use such weapon as long as it maintains the game balanced. However, M4A4 / M4A1-S was almost inexistent in comparison with the presence of AUG and it clearly shows that it broke the balance of the game in terms of weapon usage.
The AUG can be used as much as it doesn't make the M4 irrelevant or close to it. I also do not like what VALVE did with TEC9 and FIVE SEVEN because now we're almost never see these weapons in play.
I don't know but it looks like VALVE can't find an equilibrium when it comes to CS:GO weapons, and that's because whenever a weapon is too powerful it gets nerfed so much that it doesn't make any point to use it. Also, when a weapon is to weak it gets too much of a buff and you'll not be sure if you're playing CS with a BFG or not.