AFAIK, all the NIK plugins expect 16bit data. When I use them in Photoshop Elements or via Nikon's Capture NXD when converting from RAW, the file I send to a NIK plugin reports 16 bits before and after the plugin is run.
In Affinity Photo I ran some NIK plugins one time, I believe in 16 bit mode because the application reported it supports 32 bits. After running that one time all the NIK plugins are greyed out. I tried re-installing Affinity but although it still sees the plugins from the previous installation they don't function any more. My other plugin hosts (Capture and PSE) still use them.
I guess if it's true that Affinity can only send 8 bit data to plugins, I'll stick to Photoshop Elements for now.
Does Affinity Photo support both 32 and 64bit plugins? I think I installed 64bit versions (on Windows 10).