Sticking another 2 cents into this discussion. I think the amount of effort/scratchbuilding/3Dprinted parts thing would only start factoring in when the judges have narrowed the field down and are trying to decide the top three. For example let's pick custom category: You have 5 vehicles that have flawless paint, no glue smears, no fingerprints, great bare metal foil, all 4 wheels are touching the ground, etc - you can't find fault with any of the basics of model building. That's when they then have to start looking at what else has been done to the model. Is this custom basically box stock versus a one that's chopped, chanelled, fabbed parts etc. That's when level of effort comes into play.
That's why when entering a contest it's important to fill out the form and point out the extra's you've done to a model. Because as much as you would like every model judge to be a walking data bank of every model kit ever produced and of every real car produced, it's just not possible. They are human and are doing the best they can to come up with fair results.