I think the issue is sealing the panels for expansion and contraction during seasonal temperature changes. Layers of shingles can expand and contract by sliding

I think the issue is sealing the panels for expansion and contraction during seasonal temperature changes. Layers of shingles can expand and contract by sliding overtop of each other and butting into each other. There are gaps in the shingle installation for this purpose and only two nails are used per shingle to keep them from buckling and splitting. The overlap of many shingles is what typically keeps the water from being able to get underneath (which will start rotting wood and shorting connections). So if you didn’t have layers of paneling like Tesla is doing, you really don’t have a roof, since it will get water under it and eventually leak a lot. Source: norm on this old house