Creating a vehicle, from a numbers perspective, it's essentially very easy. Just research the numbers, and type them in, right? Sadly, a lot of the data is very difficult to research. It's how you make up that unknown data that separates a good mod from a bad one (assuming the bad ones even tried). This is where knowledge, understanding, and simply driving it comes into play.
It's the sims that let you play with too much of this unknown data in the vehicle script (e.g. tyres) that gives people such a royal capacity to screw it up. In the case of the LFS tyre model, the technical parameters that matter are generated from simpler, easily researched values (i.e. LFS currently just uses dimensions, which is very limited, though the new model, should it ever go public, is set to address these limitations). This is limits the accuracy of the vehicle but at the same time, means it should at least be vaguely correct in this department.
Other sims give you full control of tyres, and lots of people don't know what is correct in this department. I worked with the guy who used to be employed by SimBin to do their handling, and the description of his learning process and the curves he made partly explains why cars just snap out in GTR (for example).
The ideal vehicle editor presents you with all the real world numbers that you can research, or measure yourself, and generates all the other data needed from their, with some advanced option screen to override aspects of the generated data in rare case you have a real value to put in.