Hi at all,
Today I had the same problems as everyone in this thread.
My future main setup for this year events consists of a MacPro with a clean install of High Sierra and several BMD capture cards that had always played flawlessly with older OSX.
Today we tried several different computers, MacBook Pros with Sierra and also High Sierra.
High Sierra had problems from the beginning. We got it on the MacBook Pro with High Sierra after these various workarounds offered here. By the way, thanks to all users here in the forum who also write about it! Clearly why it suddenly worked after that was it not.
Now the MacPro stays over. This machine still does not recognize the devices properly.
The only option at the moment is the following workaround: Here it is not necessary to disable SIP or Gatekeeper. Here was the way to set up a root account and run through this installation as some threads described above
After restarting into the normal user account, unfortunately, High Sierra still can not recognize a single device.
Here is currently only the way to switch into the root account and start desktop video. Here is every device recognized. Then log out of the root account and log into the normal user account. Then every BMD device is recognized.
After a reboot and when you start in the normal user account again, desktop video recognizes not a single device.

What still helps, I have found in the meantime: It does not even have to be a root account. A new admin-account is also working. But it is always necessary to log from the user-account out, switch to the admin account, then start desktop video, then log out, then log in the normal user-account and then desktop video recognizes the devices. Unfortunately this way is not great.
Today I phoned Blackmagic Support and nobody can help me here. It is referred to Apple.
Maybe someone found a another workaround so far.
kind regards
Daniel