Im using several Asus Z270f boards with 7 cards each, without a hiccup. Just turn on above 4G decoding, put all pcie to gen1 and disable everything you arent using.
I am using the same board with 1080ti but I'm having problems with the 7th graphic card.. It detects 6 but not the 7 for some reason (tried with gen 1 pci-e but no change). Any advice?