So I ordered both the AB350 Gaming K4 and the AB350 Pro 4. I got the Gaming K4 in yesterday.
In my tests; only 4x Nvidia cards worked with Ubuntu 16.04. I tried a lot of different bios settings; there might be one I missed, or a future bios update that enables 6x Nvidia: but for now these aren't worth it IMO. Also might work with 6x amd cards; I only have 4x 470s, so I can't test that.
as i suspected these motherboard are not for 6 x gpu not good for gpu mining, also without the double molex there is risk to fry something with gpu that have high consumption