There seem to be multiple issues at play:
- NaNs in directionality mode. That could be caused by incorrectly authored normals/smoothing groups as @SF_FrankvHoof has pointed out. Inspect the meshes in a DCC package, or let Unity generate normals for them automatically in the mesh importer.
- Make sure that lightmap UVs are properly laid out. You’d need to do it yourself in a DCC tool, or have Unity generate them for you.
- As for validity artifacts; I’d highly recommend you’d check out the following chapter in the troubleshooting guide: Chapter 15: Invalid texel artifacts
- Lightmap UV seams are very common when using modular meshes and are very difficult to get rid of. Again, I’d recommend that you’d check out this chapter in the troubleshooting guide: Chapter 10: Lightmap UV seams