I've seen a number of comments in regards to how to fix issues with old mod meshes from Skyrim that don't work correctly in SSE. The comments basically boil down to "you need to fix the mesh in nifskope", but none of the comments actually say how you go about doing that fix in nifskope. Does anyone have a run down?
Example mesh that doesn't work right when just dropped in: Insanitys Celtic Coin (available on Skyrim Nexus).
I have a number of my own meshes that don't work, so I'm trying to get this figured out.