These crossbow pistols are in general mostly utter garbage. Could they help you in a survival situation, sure! However, if you are willing to carry a little more weight you would actually have a real crossbow that would deliver a lot more punch and be infinitely more effective. Still, if you are going to carry that kind of weight just pack a rifle!
A full size crossbow is larger which not only makes it heavier but also unwieldy. The main reason somebody might choose a crossbow pistol over a big crossbow is not weight but because the crossbow pistol is much less awkward. Also the crossbow pistol is easier to cock, I know, I've got a full size crossbow. And the crossbow pistol is much quieter than a rifle.