My Odroid-C2 runs a mainline kernel. I use it headless, so I'm not sure about graphics support, but its been otherwise extremely stable. It did take a year or two after release to get full upstream support. I'd imagine this new board will follow a similar trajectory.
But, I dont have one to test - it certainly might ship with a weird hacked up bootloader or something. I do have a board based on an earlier generation Amlogic SoC that I'm quite happy with. Worked well out of the box and has continued to do so.
