Just thought I'd post a little update, as I am still trying to resolve this issue... in case someone comes across the same issue down the line.
I tried using my intensity extreme card with my macbook air, and everything works fine, my camera is recognized as a webcam, and I get my feed as expected in Chrome, and inside Streamyard.
The difference between the two machine is the OS (mac pro is running High Sierra, Macbook Air is running Mojave), drivers (mac pro is currently on BVD 10.9.10, whereas Macbook Air is running latest 11.5) and obviously BM hardware with the mac using pci Decklink Duo and the macbook air using a thunderbolt intensity extreme.
I suspect the issue is with the drivers, possibly the decklink firmware as well. problem is I can't just update to 11.5 in one go, when I do that, BVD tells me there is no device connected. see this previous thread viewtopic.php?f=18&t=109738&p=605631#p605631
I had to roll back to 10.4, and now I have been slowly upgrading one version at a time (other wise I get the no device error again, I tried), I think there is probably a firmware update bundled in one of those update that I am still missing for 11.5 to recognize my device. Eventually I will get to 11.5 and maybe that will solve my issue... maybe