Have a few 6 and 12 core cMP upgraded from 4.1 to 5.1 with Mojave and Catalina. Using original and flashed 680 with different memory configurations. All working very well, unless you are using CUDA, as there are no NVIDIA drivers for this OS any more, only the built in Apple drivers.
Are you using a wifi card in the cMP? Not all of them did work out of the box. Search this forum for tons of details and great Guides.
btw.: Also have a look at the ROM thread, if you want to use the latest ROM version...