I'm not an expert, but did you export your terrain from MED? If so did you alter the ambience? I did the same this and my entire terrain is too lit up and looks cartoony, until I realised that you can adjust how much the light affects the terrain in MED (A6 as far as I know). Now my terrain has low ambience and is only lit based on the intensity and distance of lights (dynamic or static) and sunlight. I hope this helps.