I recently acquired a Kobol Helios 64 (Introduction - Kobol Wiki). It has a AArch64 architecture and is powered by a Rockchip RK3399.
I wonder if it’s possible to install NixOS on it. The Rockchip RK3399 is supported by the community in different boards (such as NixOS on ARM/Firefly AIO-3399C - NixOS Wiki). Is it worth a try? Does anyone have some experience on the subject? 
It is probably worth a try, generally try to follow existing work on the subject.
Stick to the wiki and (if available) existing nixos-hardware configuration options to guide you.
Based on my limited experience with arm/x86 boards with quirks, you can get it done, but it might take a bit of time. If there is prior work on the SOC, then someone hopefully got most of that done though.