As reluctant as I am to come crying about how something is OP....
SMG is OP. At least, as far as zombies mode goes.
The issue is, since .75, the smg does drastically increased damage in this mode. Before, it was certainly the best weapon choice-zombies take reduced damage, so the rifle and shotgun don't have the RoF or magazine size to kill a zombie before he noms on your delicious brains. So, your best option was to grab the SMG and hose the zombies down and pray they died before they reached you. There was a delicate balance-humans could group up and blast hordes out to reach intel, or the zeds could force the humans to scatter and pick them off, preventing humans from reaching critical mass.
Now, it takes maybe a half a clip to two thirds of a clip to kill a zombie, leaving you plenty of ammo left to hose down his friend, and since killing a zombie completely refills your ammo, there's never any reason to be conservative-just spray them down at long range; they'll die eventually, and you'll get all 100 of the bullets you used back.
This has lead to a very unfortunate process: human team pushes up, stands on top of a the tower closest to the zombie base, and hoses it down with automatic fire. Zombies can't leave past the withering curtain of bullets; their only option is to dig tunnels for the intel, which is such a time consuming and unreliable process that it gives the humans an uncontested win.
Now, it used to be that the best way to combat uneven power balances like this (say, when half your team quits because they're losing/can't figure out not to use guns/humans are teamstacking/etc.) was to dig a hole down to bedrock for the intel and whack and fool dumb enough to drop into melee range with no place to run, buying the rest of your team time to dig a tunnel or something to push for a cap. But even THIS doesn't work anymore. Several times, a human with an SMG as dropped down onto the intel and come face to face with me and my shovel, and they react by hosing me down with bullets and killing me.
Let me reiterate that: A human, with perhaps one or two blocks of space to move in, has become stuck in melee with a zombie who was waiting for them, and won. That should not happen, ever. Despite the ability to superjump and heavy damage reduction, zombies are still hard-pressed to win: On open ground, zombies lose to smart humans every time, which means they have to force humans into melee by digging tunnels and utilizing corners. Stairs kill zombies, you can't sprint to close distance. Cliffs kill zombies, humans have perfect vision of you coming for them and can kill you before you can reach the base and jump to the top. If humans win in melee range too, I really don't see the point of the game mode anymore-there's no contest, just humans endlessly stomping the few people stubborn enough to stick around after getting stuck on zed team.
TL;DR: SMG needs a damage nerf in zombies mode, or buff zombies, or stop giving humans all their ammo and hp back on a kill. Anything to balance the teams out again.