I can't speak for the electrical eccentricities of NuBus, but I have found that PCI Express does not like long (unshielded) ribbon cables. Anything over approximately 8 inches tended to cause issues for me.

I do see that this exists: viewtopic.php?f=16&t=14915

However, I don't see any sort of cable length. A shielded ribbon cable might be necessary.