I forced myself to play assault more and it can be done. It just gets frustrating sometimes on the bigger maps. You see someone 50 yards away and don't bother to shoot because you'll never hit them with the smg.
On big maps you kind of have to play the sneaky bastard. Sneak around and hide in a shell hole or something and suddenly the flag starts to cap and everyone is looking for you.
I really wish they would add a rifle option to the assault class. Maybe a bolt action or something without much stopping power. The class is kind of balanced out by their very useful gear but of course in some situations, assault is just the wrong class.
I've actually never used the shotgun. The SMG is fine, but obviously very situational.
I am currently playing assault and haven;t noticed much of an issue. Am still using the default SMG and been doing perfectly fine with it. Most games I end up with a 15 or better K/D ratio. It isn't like I am playing in a special way either, am just playing like I do with all classes.