In RC, they use it for rapid prototyping to verify that everything is working properly before they make the molds of the parts. However, 3D printing is not strong enough for part like suspension arms and shock towers. In short, 3D printing is good enough for prototyping, but not for making actual parts for these vehicles. Of course it depends on application. For certain parts, it would be too weak, but in parts that are not abused, it could work out. Many major manufacturers use 3D printing in their vehicle prototypes, but they don't use it for the production vehicles.