The biggest hurdle Apple is going to face is the UI and responsiveness.The touch screen isn't made by Apple. Apple can't guarantee the fast and responsive UI as it is on the iPhone and iPad.
The problem with car headunits is that, unlike a phone, they serve a much different purpose.
Capacitive touch for example isn't really that good a feature in a car. How many people wear gloves when driving? I know i't sbeen on average -15c for most of this winter. I drive with gloves on. You can't use a capacitive touch display with gloves on... and since its the car, you kind of have to for certain things.
you also don't want the headunit to require fine motor control to interact with. its a car and you're driving while using it, meaning the less attention you need to do give the display while interacting, the better. pinch to zoom a map while driving? thats too much attention IMHO.
Car displays have to be usable by covered gloved hands, as well as be primarily 1 digit.
So, they're likely all going to contain slower and more cumbersome touch input... its just the nature of the beast.