Volume knob... depends on if your tablet supports USB host and if Android recognizes the device and if android software knows how to use it. So probably not.
Steering wheel controls... probably not with a Tablet.
Ext HDD... requires usb host. I'm not even certain android supports such configurations.
Same amount of wires. The only difference may be the device placement location. If you can fit the hardware in the dash, it may require shorter cable lengths.I know a computer can do all these with additional components, but with a lot more wiring.
Right now I've got a tablet in the car running android (which I will replace with real Linux as soon as I can). It's okay for music and nav. Integration is poor though. Android was not designed for your car.