I mainly play Engineer and our weapon of choice Is the submachine gun, and, yes, they are very good ... but, overpowered? No, absolutely not. A Sniper Rifle can kill you instantly. An Assault Rifle can kill you in three hits. A Submachine Gun takes longer. So, by any standard, it is not an overpowered weapon.
At midrange if any weapon is considered over overpowered it is the Assault Rifle. And I don't have a problem with that because the Assault Class are supposed to be Frontline Fighters and they need the weapons to back them up.
In addition, it is not possible to deal with a long range sniper with a submachine gun or an assault rifle. They have you dead to rights. Long range attack is their domain.
Also, at close range it's a toss up whether the assault rifle or submachine gun will kill you faster. They are both deadly and can hipfire very well.