The best answer is to check the motherboard Qualified Vender List for memory compatibility. Early X99 RAM problems were detected by Eric and reported on this forum.
For instance if you search Newegg there are no 8 GB x 8 DDR4 kits offered at any speed, buyer beware!
What is a DDR4 Kit?
With the latest bios on the Asus Deluxe board the 2666 ram runs fine at that speed provided you make some changes to Gskill's XMP profile. They really chose an odd way to profile that ram and it's not a recommended config.
When you say that it's not a recommended configuration, which of these three are you referring to?
The setup in the picture above? (In Bill Gehrke's Post)
64GB (8 x 8GB) , PC4-17000, 2133MHz (G.SKILL - x99)
64GB (8 x 8GB) , PC4-21300, 2666MHz (G.SKILL - x99)?
I would recommend the 2666 models even if you clock them at base 2133 profile. Those have to be far better grade ram.
However if you set them up right 2666 is great performance