Does anyone here have an in-depth knowledge of this system?

Alpine made a USB interface that allows various devices (Ipod, USB flash drive etc) to be controlled from the head unit, but apparently the Alpine unit had some issues.

Third-party units have been made and sell on Ebay, but my model of head unit isn't listed as being one which is compatible with these units.

I don't understand why this is so. Surely if AI-net is a universal 'system', these units should work with any AI-net head?