I leave a slim 32GB USB flash in the port, and it works fine. I can browse/show the playlist pretty quick, and it resumes correctly in the middle of the song everytime I turn on the car.
I think the problem could be about the timing when the iPod becomes available to the system, or how it presents itself to the system. I think the system identifies if a drive is the same as last time based on certain device ID/name. Also, in order to resume where it left off, it probably needs to get the whole file/directory structure of the USB device, which the iPod might not pass to the system right-away/correctly. Another possibility is if the iPod has an internal magnetic drive that takes some time to spin up (unlikely).
Everytime I plug my LG phone in to charge, the car does detect it as a USB device, but the phone doesn't allow accessing its storage until I unlock its screen, so that's another complication.
Anyway, it's probably easiest to just grab a (slim) flash drive and copy only your favourite songs over.