I know this thread is many years old, but I was also having the problem of some static meshes having very dark surfaces:
And inspired by this topic I tried using Flat Shading instead of Smoothing, but that was not the solution (and made the models look bad as expected). Then I reviewed my maps and noticed that due to a bug in my material, Ambient Occlusion was being set to 0.
As soon as I correctly used the AO map texture in the material, things got back to normal. I.e. make sure AO is not set to 0!
