Trucks are larger and heavier. That means they take a lot longer to stop, so you have predict things around you further ahead. Also, you can't jackknife a car - I'm guessing the physics of driving a Mac truck with a heavy trailer and 18 wheels is a lot more complex.

But the physics (mass of the load/center of mass of the truck load system, their moments of inertia, N on each of the tires , etc) can be factored into the equations and solved in real time, no?

How far is the line of sight for google self-driving cars?