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.

I expect the Pro 4 to have similar results.

Used this CPU:

https://www.newegg.com/Product/Product.aspx?Item=N82E16819113437

and this RAM:

https://www.newegg.com/Product/Product.aspx?Item=N82E16820233982