Rockchip is currently using an older 4.4 kernel codebase and things are in a "basic functional" state; it works and we've learned lots about RK support but it's not perfect and we don't plan on developing it much further. In the near future we will rebase our Rockchip support against the mainline Linux kernel - probably after 4.20/5.0 is released. This will be initially less functional than the 4.4 codebase, but once we start finding and fixing things on mainline we can upstream the fixes directly to the kernel and we don't end up carrying a huge patchset that's at a different state to other hardware platforms. Rockchip video decoding also needs to make the evolutionary leap from using an ffmpeg hwaccel to a proper stateless V4L2 decoder so it's using the same Kodi code-paths as everything else - and that's quite a bit of work. So Rockchip images will not move to Beta for LE 9.0, but the goal is to have everything sorted for LE 10.0 (Kodi v19) latest, or an interim release if possible.
TL/DR; RockPro64 is a nice board but you should not expect the same level of maturity as the Raspberry Pi for a while.