Seems silly asus won't support the x399 with the simple task of enabling us to use resizeable bar, when msi, an ASRock have. Buddh of mine is running resizeable bar with a 6800xy on a x399 Taichi problem free an a nice performance boost on some of the games he plays. These were expensive boards, I don't think it's unreasonable to ask for asus to offe a little support on the enthusiast platform that isn't that old. I guess they no longer want my money. Never considered asrock till now.
I was reading some of the PCI-SIG's documentation on it (it wasn't the most exciting, but informative). It seems that BAR may not be applied to all motherboards and it's due to the size of the UEFI/BIOS. ASUS has to be careful when adding Smart Access Memory to older PCIe 3.0 boards it seems, or the testing would still take some time since BAR requires access to a very large area of memory.