[bizuka]

well i'm not a specialist, but i have a 905x and a Rock64.
The rock64 can output 4K HDR exactly like my tv does without a client, all the brightness, its just gorgeous (sometimes the sky shots blinds you!) meanwhile the 905x cant achieve that kind of brightness, (its still ok tho) but its not a perfect experience
i'm sorry for not being "technical"
the devs on the coreelec forum told me that the 922x outputs exactly (in terms of image processing) like the 905x so.. 80bucks for nothing? only for 1080p soft decoding damn that was a good marketing strategy by them!
tested on a samsung ks7000 connected to a marantz 6011.

What OS are you using on the Rock 64 ? 

i'm using libreelec nightly builds.