What is the risk in unplugging a USB device without disconnecting it in the OS?
USB is intended to be electrically hot-pluggable, so the issue is actually one of software state rather than at the hardware level.

Essentially, the concern is that a mounted file system could have uncommitted buffers in ram at the time when the device is unplugged, or could be in the middle of some interruption-unsafe move operation or meta-data modification. Journaling file systems can provide some protection against this if well thought out (or make an even more hopelessly confused mess if badly designed). Further questions about that aspect of the issue would probably belong on superuser.

It would certainly be possible to (accidentally?) design a USB flash controller IC which conducted interruption-unsafe housekeeping operations below the level of the SCSI-like block device interface seen by the host, such that even when the host OS thinks the device is safe to remove it might not yet be. Hopefully that is not the case.