Tesla's steering wheel thumb buttons are also part of the equation. The left-thumb scrollwheel is I think hardwired to audio volume and the right-thumb scrollwheel is soft state, which you could assign to the fan speed, among other things. That lessens the need to touch the screen for common stuff.

In general, you can evaluate usability by breaking down a series of necessary steps (look for the button, navigate your finger to the button, press the button) and you can estimate the timings with remarkable accuracy (Fitts's Law is awesome). If I give you hundreds of little hard buttons, the time doesn't get much better. The empeg, on the other hand, doesn't require you to look for it. You can just feel for it and you can (eventually) remember the sequence of down-left-down-right codes and do the navigation without reading the screen.