Yes, which is why SMGs are the class weapon and why we also shouldn't have an open weapon system.
But since we do, we could go about balancing it out slightly.
The signature weapon buffs while a nudge in the right direction aren't all that impactful. Sniper rifles and LMGs being the two strongest, with SMGs, ARs being fairly meh.
Something along the lines of you select a class weapon you get the signature buff and 100 attachment points.
You select one of the open weapons, ( Shotguns, Carbines and DMRs ) you just get the 100 attachment points.
Using another classes class weapon you have only 80 attachment points at your disposal. So you can still use it, but it won't be as effective as it would be with it's proper class.
This is a nice middle ground between closed and open weapons.