CNET took the Echo Auto for a test ride, and they came away disappointed.
One of the most significant downsides is that asking for directions from Alexa doesn't work without having to tap on a phone to confirm. That is the essential feature while driving. If you have to take the phone out, you might as well ask Siri or Google for directions.
Loud wind noise can also overpower the microphones on the Echo Auto, making it useless as seen in the video: