I guess it depends how "embedded" the target is, I guess. I'd rather write an xmodem implementation in assembler than I would zmodem. I'm envisioning that you're using xmodem to transfer "brain-stem" type firmware where you're talking to a very constrained system. (I know I've sent firmware to Ethernet switch bootloaders via xmodem over serial. Presumably I was very, very close to the metal in that case.)