Damage structure, Range, Accuracy, Hip Accuracy, Reload times...etc.
Smg's got less stopping power than rifles and with a slow dead someone can perhaps still kill you after you put a round into someone that will eventually kill him. Requiring you at times to put additional rounds at enemies
Smg's got lower bullet velocity than rifles meaning that over distance the bullets drops quicker.
Smg's have less barrel accuracy than rifles, sure you can hit a standing target up to 150 meter probably. But most people will be fighting with a very low profile from behind cover.
In Red Orchestra a rifleman can kill by placing a bullet nearly every where on an enemy soldier from a safe distance. If one bullet when hit pretty much always kills you do not necessarily need a lot of bullets.
As long as Red Orchestra got good open areas with perhaps less stuff blocking longer range vision (especially on roads etc). Then a rifle will continue to be a strong weapon.
Remember that back in the second world war there was a reason why soldiers were not only equipped with smgs but actually with rifles as well. If weapons obtain their realistic properties then that realistic balance will form.
SMG's have clear weaknesses, if balance is your concern then Snipers, MG's, Semis and Assault rifles should be your concern. And luckily all those classes are extremely limited in their amounts.