hardly a drawback is it? but tbh we dont know the details or how it would work. To me it seems you can combine parts from various weapons

Fallout 3 also includes the ability to create your own items using a crafting system, as well as combine two weapons to make one stronger or more durable.
Create your own weapons by gathering resources from fallen foes. Weapons will deteriorate over time and with use, so maintaining your arms is a constant consideration
^^ taken from the FAQ on nma-fallout. Everything seems good thereYou'll occasionally find or be able to buy schematics for new and unusual toys which can then be built using parts scavenged from the world. The best example of this is the Rock-It-Launcher: a jerry-built projectile lobber that can fire rocks - or any junk you may have clogging up your inventory
