I was having trouble with landscape shadows producing weirdness like this.
I spent 3 days trying every solution I could find here on the forums or answerhub.
Finally I discovered that the problem almost completely went away when the material was set to two sided.
There were still a few splotches here and there, it got even better after setting 2 sided lighting on the lightmass settings.
Just figured I would post here in case anyone else was going mad with the same issue.