With the current RK3399 release I have two issues on my RockPro64:
1. HDMI audio passthrough is completely broken to my Denon AVR-S740H; it sounds like the AVR is treating it like PCM (i.e. just white-noise)
2. The sound randomly cuts out in videos for anywhere from about half a second to 3 seconds. It can go an hour without doing this but then do it several times in a few minutes, but I've not made it through a 2 hour movie without this happening.