I think I figured out the problem. I checked just one, the helicopter, but I think that this information applies to all the nif problems. I think that the program you used or somebody else used to export the .nif caused some errors. I export with Blender 2.49. I imported then exported using that version and the file works in game now.
If it was a texture problem, the unit would show up and just be pink. If you look at the file structure of your nifs compared to some in game versions that work, you will notice yours only has NiTrishape Nodes.
Are you using Blender, 3DMax, or a 3dmodel converter to create the nifs?