Recently ran into some fbx import errors with Unreal regarding degenerate tangent bases, and nearly zero binormals/normals. You can research the real meaning of these issues for a better understanding of the root causes but I'm just gonna post the solutions that worked for me after trying a bunch of the options listed on forums around the internets. My models were built in 3ds max and showed no problems. Well, not entirely true. My Occlusion mapping/Lighting passes were not coming out right. In the process of modeling I had attached some parts that had been mirrored. I think this was the main source of the issues I encountered later and was fixed (mostly) by re-importing the fbx model after export before anything below.
There are many posts about this online offering different solutions, depending on your DCC package. With Max, I found the solution involved just a few things. Obviously, you need proper UVW Unwrapping. Assuming you have this, it's a good idea to "Reset Scale". Next check for degenrates et. al. using "Modifiers/Retopology Tools/Mesh Inspector." Run the inspector and fix all errors listed in the Modifier. Now Export FBX with Tangents and BiNormals in FBX exporter. I usually use Triangulate as well.
On Import into UE: Uncheck remove degenerates (especially for small objects).
Import Normals and Tangents.
This should get rid pf the errors upon import into Unreal.
But...... there are always some other factors that could be afflicting your goemetry and for that, I wish you good luck with the other solution posted around online.
