Phoronix reader Ching-Nung Lin has sent us over a link to the ASUS de facto list for figuring out ASUS motherboard compatibility with Linux and what distributions/versions are supported with each particular product. He also included his communications with ASUS' customer service manager Mrs. Hong and product manager Mr. Chen.
Ching-Nung Lin was seeking help for a BIOS issue with his newly purchased X99-E WS motherboard, but because he was running CentOS 7, ASUS is not going to actively work to address his BIOS issue. For their support list, they only mention "RedHat 7" and Ubuntu 14.04 for this particular motherboard. Even though CentOS 7 is effectively a rebuild of Red Hat Enterprise Linux 7, to their customer service that appears to not count.
If you want to see the ASUS motherboard Linux support list, visit this ASUS.com PDF file.
On many of their motherboards they test Fedora, openSUSE, Red Hat (Enterprise Linux), and Ubuntu, but for many of their desktop motherboards they may only end up testing out one or two of these distributions. Additionally, they just verify the initial release (or LTS) release of a given distribution and not the latest up-to-date versions. The motherboard list also doesn't appear to be completely exhaustive as there are very few Skylake boards currently listed on the page.
So if you plan on buying an ASUS motherboard and are worried about having any technical support from the company, make sure your motherboard is on the list and that you intend to use one of the mentioned Linux distributions or at least willing to try out one of them in case you run into problems and want to contact customer service... Otherwise they recommend you try out a supported version of Windows, similar to other motherboard vendors recommending you use Windows to solve Linux motherboard issues.
For those looking toward desktop motherboard recommendations as a Linux user, as I've mentioned more than a few times, lately I've been buying a number of MSI motherboards. Aside from the fact that MSI has been sending out a number of review samples recently for specific Linux testing at Phoronix, they have been working on UEFI/BIOS updating from the Linux desktop, have been very open and communicating with me about Linux on their products, etc. Most motherboard vendors don't really have much interest at all in the Linux desktop outside of the server/workstation space. On a related note, if looking for Linux-friendly graphics card AIB partners, here are my recommendations.