Fu and Cai are one of the top Md if not the top MD this generation.
But having look at their style and play, and the fact that they are struggling to dominate world MD and having to lose so many times to veteran pairs like CTW-LWW , Eriksen-Lundgaard both of whom werent so top in their prime, I believe that Fu-Cai cant match Tony G-Chandra V, Sigit-Chandra, Ha-Kim, , rexy-ricky or even Cheah Soon Kit -Yap Kin Hock in their prime.
Fu-Cai lacked depth, they are fast and powerful but their game lacked technique and variation. They are at the top now because the other young pairs are not as good as the pre-2004 top pairs in technique and depth except maybe KKK-TBH.
Post 2008, Fu-cai would definitely slip down the ranks as their power game cant last Cai Yun age and without depth in technique they may not win many tournaments after 2008.