If using google for your navigation instead of the inbuilt one you loose the feature for the car to display road signs, in addition to this the car will constantly warn you that you are exceeded the speed limit as it is no longer reading the limit of the roads.

This would almost suggest that the camera to read signs is also comparing the information to the sat nav so if not using it it doesn't work.

Also the hud doesn't provide any turn instructions, I wonder if there is an api that can be access from the phone to display this