The only place you see the instructions are on the phone itself or the display of the car. The build in shows the next instructions within the cockpit & HUD.

It has no access to the wheels, and in tunnels the GPS will simply not work where as the build in sort of does.

The build in depends way less on healthy Internet access (yes there are parts of Europe that I drive though with cripple G2 at best).

Mind you, I SWEAR by Google maps for driving (in Europe) since it is so current and predicts / reroutes better then anything else I've ever used.

Paying major money for updates of the build in also is a real pain.