HI all so I brought the ultrastudio pro https://www.blackmagicdesign.com/au/pro ... studiousb3
which has fairly much everything in and everything out (the breakout cable is redick) and it captures sdi fine over USB.
It is only in the last month or so that Blackmagic USB3 capture devices have supported OSX. I dare say the UltraStudio SDI should also work fine too. ( i only got the pro version as someone was selling it cheep on ebay)