For those who are struggling to tackle this problem, there can be several reasons why.
Here are a 2 solutions that might improve the quality.
Solution 1 - Your scene was created with a scale too big or too small. Attempt to resize your scene keeping scale reference to Unity's [Assets] create cube.
Solution 2 - (my solution) The models I had FBX exported from Maya were Combined Models. Even though the textures read fine in Unity, the light mapping did NOT.
Make sure the models are 1 solid geometry with even triangulated geometry.
Also when exporting your assets from Maya, in FBX export [Options] [Presets] you can include the check box [ ]Tangents and Binomials. Then when in Unity, after you import the model, look at the models setting and Import [Tangents] and Import [Normals]. This was discovered as I took models through my Substance painter workflow.