Is crash safety. Don't get me wrong, the attempt to strap solar panels to a "car" in an efficient way is admirable. But, the main approach to making the design tenable is through improved overall efficiency, a big part of which is weight reduction. That's all well and good, except you're going to be compromising structural strength in an effort to reduce weight. This would be "fine" except it also compromises crash safety, which is a much bigger deal. Yes, you can talk about lightweight components and yes they help, but only to a limited extent. Even putting aside the structural issues, there's the matter of what happens during a collision with a traditional, much heavier vehicle. Physics dictates the lighter car will be the one to go flying. It sucks but that's sort of how it is, unless the safety "incentive" to maintain mass is slowly eroded across the global fleet