I would argue the biggest issue with the "miles driven" metric is that it isn't contiguous. Your test drivers can handle all the rough stuff, and your mile counter ticks up on the straight shot highways. (Arguably, you could be super dishonest and handle every single intersection manually, and still have a massive miles driven statistic since most distance is accumulated on straight-ways.)

Disengagements is a useful number, and the disengagement numbers say so much more about the real state of self-driving cars: That they fail as often as a non-synthetic oil change. And if humans had driving system failures that often, we would not be considered good at driving.

The longest number a company should be marketing is how many miles they've driven between disengagements. Like the "days since an accident" sign at a workplace: Fail, and you go back to zero. The numbers will be smaller, but they'll be a lot more meaningful.