I built a system using this board last week and I'm writing this comment on it right now. It's still early, but I like it so far. I got it because it was one of 2 mini-ITX boards that had both USB 3.1(A&C), and an M.2 slot. And it had about double the USB 3.0 ports that the competing Gigabyte board had.
I notice you were using the P1.2 BIOS. When I built this, the current BIOS was 1.5 and one of the major parts of the updates was memory compatibility. I wonder if you'd have any better luck with your memory issues with a newer BIOS. I'm using Ripajaws V @ 2800MHz with a locked i7 6700. It took some fiddling to get it to boot with both sticks in, but eventually it worked with the default XMP profile.
I've had the board for almost two months now, and have almost no complaints. Like you, I'm using the locked i7 6700, and I had issues initially getting the RAM OC settings down, but once everything was stable, it worked great.... as long as I don't mess with the settings anymore.
Couple of issues though... I really wanted to use the Ultra Fast boot (the setting that skips the bios completely), to try and get system boot to sub 3 seconds (it's about 6-7 seconds now to desktop), but when that setting is enabled, anytime I attempt to go back to bios using the ASRock "reset to bios" tool, I can't boot properly again without resetting all my bios settings. Same problem experienced when using the windows "reset to bios" setting as well.
Additionally, the little piece of bent metal that the Bluetooth and WiFi antenna connectors are attached to is useless. Mine is partially broken (one of the connectors is no longer locked in place and spins unabated), and the metal is not bent correctly in place. It is slightly shifted to one side, but that's more of a minor annoyance I guess.
Also +1 on the fan headers comment, also had to get a splitter.