I don't understand the hassle about grease guns.
In the end it only requires 2 or 3 zombies to overrun grease gunner.
If you compare grease gunner to a good rifleman, someone with skills can take out 3 zombies easily if he can aim.
And I don't think they should be nerfed. There must be a way to kill zombies and this is just one way to do it. If we nerf them, then we don't have a single good smg. You can't make everyone use rifle or carbine. Not everyone wants to.
I say leave them be.
When there's only 1 or 2 guy left with a grease gun, with 3 zombies for each, then yes. Otherwise, the balance is not respected. It's more like 3 grease gunners for 1 zombie, which means the zombie dies (except if he's really lucky and manages to get one when he reloads, which happens extremely rarely).
Obviously you haven't played when korefan is there. Alone against all the rest, he can survive 2 more minutes because of that weapon.
You say we don't have any "good" smg. So for you "good" = extremely powerful, that can kill with 3 shots the first zombie (2 hs + a pistol bullet in the feet kill it).
And people can use smg, just not that powerful! They just have to spray less and aim a little bit. Won't kill them.