I have played only a little bit (won one game), and for the most part, the manual does a good job of describing how engine power/weight ratios work and how that affects vehicle performance. I have, however, noticed three things that bother me about the way model design works.
1) When designing a truck, there seems to be no clear connection between engine power of a truck and the equipment it can tow. That is, it doesn't matter how heavy your artillery is, as long as your truck has an overpowered engine relative to its own weight. This begs the question, why would one have artillery guns in smaller calibers, if you can just mount the biggest ones on the back of a basic truck? Ammo usage? My suggestion here would be to tie this in together.
2) When adding wings and other parts to a plane, I have little idea as to what each component adds/subtracts from effective range. It becomes a guesswork of trying different combinations before one sticks. Could the manual expand on how individual components affect the final design? It is nice that there is a viability check though, with a summary, which brings me to my last point:
3) Could we have a basic summary, like with planes, that would predict approximate unit stats like movement penalties and firepower (assuming average design rolls), so we can go back to the drawing board like with planes? It could make model design clearer for newer players, and allow us to compare a potential new model to the old one easier.