While the communications between the PC and the USB chip use protocol that can in some circumstances correct for errors, then the serial data to the USB chip does not include anything that would correct errors.Like any transfer of data it can be subject to errors.
there's no actual serial hardware involved, RS-232 or otherwise, on either end.
There it's not a USB to serial converter chip but a co-processor (ATmega16U2) which is doing that
So what do you think the difference is between this and a USB to serial converter chip? They are both software based processors that do an actual conversion between real serial and te USB protocol.
I think, given the OP's question, this is relevant and not being covered in your answer.
I don't think it has any relevance to the OPs question, just because one has it's code in a PROM and the other in Flash makes no difference to anything to do with the conversion process does it?