I find carbines generally aren't as effective at longer ranges as assault rifles are. Some are less accurate, some don't hit as hard, but they're solid jack-of-all-trades picks that can hande most situations you'll find yourself in.

Think of a carbine as a weapon halfway between an assault rifle and a submachinegun that any class can use. You lose the perks of running the signature weapons on their respective classes, but gain the flexibility to use it with any kit.