I also have a Steelseries keyboard and mouse -- Original Apex and Rival. I don't think this problem is specific to ROG boards -- I've had similar issues with the same two devices plugged into a Mac Mini (late 2009 model) and a Z61 ThinkPad.
On the Mac Mini, the mouse would sometimes fail to be detected at wake from sleep but the keyboard was fine.
On the ThinkPad, the keyboard wouldn't fully illuminate during POST (only the main keyboard and the numpad section, none of the F-keys, and then would be fine once Windows booted.
On my Maximus VIII Hero, usually the keyboard and mouse will be fine (and always are fine in BIOS screens) but once the USB bus resets before booting Windows, sometimes the keyboard will come up only partially illuminated again. Sometimes re-plugging it will fix it, other times I must shut down completely and restart. Also, the mouse will wake the PC from sleep fine, but using the keyboard will cause it to be dropped completely.
So on every computer I've connected these two, there have been various issues with how they interact with USB. I tend to think that Steelseries still needs to work out a few issues in their firmware. Perhaps someone's discovered a useful tweak in the Asus BIOS that might help USB detection and behavior?
I'd like to try re-flashing the keyboard's BIOS, but all I've encountered is auto-updaters in a few of the driver updates, and once it's done flashing, there doesn't seem to be any reset/re-flash options. Overall, I like this keyboard and mouse a lot, but I'd love to see a more robust utility for managing the firmware.
If there are any other Steelseries users out there with more ideas, it would be great to try a little brainstorming.
Motherboard: Maximus VIII Hero
Processor: I7 6700K - 4.9GHz OC
CPU Cooler: Corsair H80i v2
Memory: G.Skill DDR4-3000 16GB
Graphics: EVGA GTX 1080 FTW2 1721/1860MHz
Storage #1: 1TB Western Digital Black
Storage #2: 2TB Western Digital Black
Case: Antec Nine Hundred
Power Supply: Antec Edge 650 80+ Gold
OS: Windows 10 Pro