The actual mosfets in the CH7 are rated 60A vs 40A in the Gigabyte. Here's the link for the pcb analysis
(asus)
(gigabyte)
It is much easier to find a stable overclock on the Asus ROG boards due to the presence of a safe boot button which will boot with stock settings and take you straight into the bios. With gigabyte, you need to remove the battery if the board fails to recover from an overclock.
As for ram support - non supported kits can be made to run with some parameter tweaks. My 16x2GB hynix sticks won't even run 2400 out of the box and they are not on the QVL list of the CH6. However with tweaking they do 3200. It just needs time and patience. Overclocking is not for someone who just puts stuff in and expects them to work with one click of a button.
Last gen gigabyte x370 boards had terrible VRM components that overheated. Thankfully they fixed that this gen. However their bios is sad. I have their z370 HD3P board and the bios on it really seems an amateur effort.
4.2 GHz should be possible on most 2700x chips while few doing 4.3 under water. Again patience is the key.