So, as I finally got my hands on a Rock64 4gb Friday, I thought I should weigh in here with tome tests, too (as the Rock64 is a RK3328 device, too).
I conducted tests with the 20180601 nightly build, as well as the "part-7" branch from Kwiboo. I also tried to compile the "part-6" branch, but that failed
to compile (something about missing GDM - I presume it has something to do with this).
EDIT: I noticed while writing this that Kwiboo rebased "part-6", I will try to compile this again now. I will also compile the current master, since this includes some neat changes.
I did test the following files, all 1080p if not noted otherwise:
BigBuckBunny 60fps, both 1080p and 4k
Anime A:
- FFProbe: h264 (High 10), yuv420p10le(progressive), 1440x1080 [SAR 1:1 DAR 4:3], 23.98 fps, 23.98 tbr, 1k tbn, 47.95 tbc (default)
- VLC: H264 - MPEG-4 AVC (part 10) (avc1) / Planar 4:2:0 YUV 10-bit LE
Anime B:
- FFProbe: hevc (Main), yuv420p(tv, unknown/bt709/unknown), 1920x1080 [SAR 1:1 DAR 16:9], 23.98 fps, 23.98 tbr, 1k tbn, 23.98 tbc (default)
- VLC: MPEG-H Part2/HEVC (H.265) (hevc) / Planar 4:2:0 YUV
Series episode:
- FFProbe: h264 (High), yuv420p(progressive), 1920x1080, SAR 1:1 DAR 16:9, 23.98 fps, 23.98 tbr, 1k tbn, 47.95 tbc (default)
- VLC: H264 - MPEG-4 AVC (part 10) (avc1)
Movie:
- FFProbe: h264 (High), yuv420p(tv, bt709/unknown/unknown, progressive), 1920x804, SAR 1:1 DAR 160:67, 23.98 fps, 23.98 tbr, 1k tbn, 47.95 tbc (default)
- VLC: H264 - MPEG-4 AVC (part 10) (avc1)
LibreELEC-RK3328.arm-9.0-nightly-20180601-19b9bed-rock64
- BigBuckBunny 1080p: Works flawlessly
- BigBuckBunny 4k: Video plays "to slow", and is out of sync with audio
- Anime A: Unwatchable because of video stutter
- Anime B: Works flawlessly
- Series episode: Works flawlessly
- Movie: Works flawlessly
Kwiboo "rockchip part 7" (a9bed38daf236ecd502357890ae2bc47a55cf9c4)
- BigBuckBunny 1080p: Works flawlessly
- BigBuckBunny 4k: Video plays "to slow", and is out of sync with audio
- Anime A: Unwatchable because of video stutter
- Anime B: Works flawlessly
- Series episode: Works flawlessly
- Movie: Works flawlessly
Other things to note which are of interest:
- Anime A did run ok on my Rasperry Pi 3 with LE 8 and 9, but had very annoying color artifacts (probably because of 10bit?)
- I did experience some problems across the builds with resumed playback - stuttering and outright stuck playback. But I was unable to reproduce this yet 
- I have no idea why the movie works, and the series episode doesn't (on 20180601) - for all I know they are encoded the same way. (see point above)
- Neither 1080p nor 4k BigBuckBunny ran on the Rasperry Pi 3 (1080p with stutter, 4k did not even run)
- The movie file works fine via NFS or USB, but stutters via SFTP. This happens on the Raspberry Pi, too - I have yet to figure out why (network is fast enough, and my laptop doesn't have the problem) (if found the reason - SFTP transfer speed is limited to 30mbit for me. I will investigate). All I know is that the bit rate peeks at 60mbit at the opening logo - which is when the stuttering happens.