UV binormals are flipped on mirrored faces.

[GTD-Carthage]

The following shot demonstrates a seam - the mirrored symmetric side of the model has normals rendering incorrectly:

The following shot shows the same model with correct rendering in Unity 3:

An old Model Viewer screenshot showing the geometry of the same ship, with tangents visible:

A Model Viewer screenshot showing the geometry of an vanilla asset from Hello Games. Compare the arrangement of green lines between either side of the mirrored object.

Check tests/cubeSymmetryTest/symmetry_seam_test.blend for sample.

[monkeyman192]

@gregkwaste I think this is more your domain