Soldato
There is zero issues with Crossfire on X79. When 7970 first came out, I had three running in an Asus X79 mainboard in Tri-Fire, worked great.
I also have a friend who is known on here as Loki who is running four 7970's in Quad Fire, without issue.
A number of people on the Asus ROG forum would seem to disagree.
Also, you said you had 3 running in Tri-fire (in eyfinity?), but don't now? Can't have been that good if you got rid of them