For all that X399 offers, it has a storage problem.
For years, we've railed about the lack of PCI Express lanes on motherboards; some people may not realize just how neutered modern desktops really are. In August 2005, nVidia (how it was spelled at the time) released the nForce 4 SLI X16 chipset (opens in new tab) for the Pentium 4 in Socket LGA775. The chipset supported a whopping 40 PCI Express lanes. Not long after, Intel realized that Nvidia was cutting into chipset sales and stopped licensing x86 to the company. In the years hence, we've been stuck with 16 PCIe lanes.
The new Z270 was a game changer with a massive 20 PCIe lanes. (/sarcasm)
Enthusiasts plug more devices into the PCIe bus when they're available. AMD's X399 platform supports a massive 64 lanes (four are dedicated to the Southbridge), and that allows you to use two video cards at full speed, add on a zippy 10-gigabit Ethernet, and even pop in a crisp new sound card if you so desire.
What the X399 doesn't give you is the ability to utilize NVMe SSDs in a bootable RAID. We reached out to a few industry sources and learned that X399 will allow users to build an array with SATA products and boot from it, but AMD hasn't employed a way to boot from NVMe SSDs together in a RAID 0 array like Intel.
We were also told that AMD is going to enable the feature, but there isn't a firm timeline. It's one thing to talk about booting from an array of speedy next-generation storage devices that can surpass 5,000MB/s with just two drives; it's another thing to actually deliver.