An emergency stop is better than nothing, but they should ALSO have an emergency "let me take control". Sometimes stopping does not decrease the danger.
Example: the waymo enters a rail crossing with flashing lights, and the barriers close with the car inside. The waymo sees the barriers so it stops. What you want in that case is accelerate and get the fuck out of there. If you have a baby in the backseat, there may not be enough time to get the baby and get out of there on foot.