It's vaguely possible, given the serial speeds involved, that SOME FTDI adapters might not work with SOME arduino clones using optiboot. The 115200bps serial rate used by optiboot is not particularly accurate on a 16MHz AVR. If (say) neither the AVR nor the FTDI had a crystal-controlled clock it's possible that you could get "unlucky" to the point where they could not reliably communicate.