why you dont like the MSI Z270-A Pro? i have this and is perfect
6 gpu is working with no problems?
yes but you need to activate something in the bios to make the motherboard check for more than 3 gpu, after that you can run even 8 gpu on that motherboard, i still like the asrock more because it come with more power, but the asus one as the M2 which is very good for removing some cables
I think you are wrong here.
6 GPUs should not be a problem without messing with the bios.
If you don´t have enough PCIe slots use USB-PCIe risers.