Miles and disengagements mean almost nothing with knowing the scenarios. It is often misleading hype. All that matters is scenarios learned and what those are. It is a myth that public shadow driving is a viable method to create AVs. That process can never come close to creating an AV. You cannot drive the one trillion miles or spend over $300B to do so. You also cannot run thousands of accident cases thousands of times each or have any more casualties especially a child or family. The answer is to use aerospace/DoD/FAA level stimulation for 99% of the scenarios that need to be learned and tested.
More detail here
Impediments to Creating an Autonomous Vehicle
Save Autonomous Vehicles companies from themselves — So they can Save Us
Waymo is leading the hype charge — That will get many more people killed than it saves