I can see the logic in using an ATX board, particularly as there is SO much more choice, and you don't pay 'SFF Tax' on cramming all those good features onto an ITX board.
The Cerberus -X makes a lot of sense with only an additional 1.5L over the mATX version, and that's from the 60mm additional height to take the longer board. If it was 7 slot and not set up for EATX, It'd be even smaller.
Space wise, I personally see ATX as as a waste of space, as I cannot think of what I'd fit in more than 3 of the slots, making mATX the perfect choice for me when ITX is too limiting.
Maybe if it was littered with M.2 slots like the underside of a Micro STX board, that would be more useful...
Height for storage is an issue for me, so a tall case is a pain to find a home for. That said, the Cerberus-X is no taller than my Silverstone
TJ-08E, (380mm) which is fairly small @ 28L.
If you reduce the volume by mounting the GPU parallel to the board, you lose the benefit of those additional slots, as they cannot be used.
Slim & tall and under 20L is good, but the shape doesn't work for me.
My cobbled together mATX case that my dev PC lives in is only 270mm tall, so barely taller than the motherboard is long; and that is fine to stash away when not in use or being worked on.
Every time I see my LAN buddy struggling with a full ATX case to LAN nights, I do chuckle and thank my lucky starts that I have discovered SFF!
I still agree that well specified mATX boards are lacking, and there are plenty of cases that would take them; it's odd that's its been passed over, with (still) too many ATX boards per manufacturer and chipset, and a measly mATX offering.
Precious few are high end and top spec for that matter.
Going back to the OP, it's interesting that the much heralded discrete (daughter) sound card on the Maximus VI/VII/VIII Impact boards has disappeared, and the new boards are conventional with a small metal shield on the codec. Only the Strix B350i and 450 in AM4 have retained a discrete card, maybe they can control noise better on Intel boards these days??