Eh, that's not a very accurate way to describe things. S/PDIF itself transmits samples of audio one at a time, whereas USB transmits frames encoded in packets,

Eh, that's not a very accurate way to describe things. S/PDIF itself transmits samples of audio one at a time, whereas USB transmits frames encoded in packets, but in practice there are going to be hardware buffers of some size, and the software will process one buffer at a time. The end result means that there are no real advantages to S/PDIF, and a number of serious disadvantages.