I was originally looking for a firewire/spdif converter. Not much to choose from. I ended up getting a usb/spdifconverter. the couple of people I have talked to one manufacture that said that a firewire/spdif converter wouldnt have any advantages over an properly implemented usb/spdif converter. That might be 1 reason that we havent seen more firewire converters on the market other than pro units since they are the ones that tend to use firewire more than usb.
A FW > Audio (digital or analog) is much more difficult to design than a 'basic' USB based interface. Thats because there is no FW audio streaming/transport standard. And no standard chip sets. All serious FW audio interfaces (Apogee, TC, Motu and RME come to mind) have their proprietry design interface design. Just like the 'better' USB based designs. That is a very expensive solution compared to a standard TI or Tenor chip for an USB interface. And considering the relatively low market share of FW, its easy to see why there no 'simple' interfaces on the market. Its simply not worth the effort.