SMG/Assault Rifle Balance

The balance may still need work, but Battlefield isn't trying to be realistic. You shouldn't expect things to be balanced like that. In real life, SMG's that shoot 9mm bullets aren't even used anymore because they can't effectively penetrate body armor. There are many more examples where it would be highly impractical to balance things around realism.

Having said that, it's reasonable to expect that an assault rifle on average has a longer effective range than an SMG does. Just like you'd expect a sniper rifle to have longer effective range than an assault rifle. Those are just broad weapon categories though. When looking at individual weapons, Battlefield has traditionally had weapons that don't really fit their weapon category. Such as an LMG that act like assault rifle, or an assault rifle that acts like a DMR.