Optimizations when using Unity Terrain over imported mesh

What advantages does using Unity’s terrain system offer over importing a terrain generated in a modeling program? Is there any specialized core support for LOD, culling, collisions, etc present or does it just generate a single static mesh internally?

Am I going to be hurting performance or loosing features (other than the terrain painting and mass tree/grass placement)by using meshes generated externally?

Yeah the terrain engine has built in mesh LOD. And you get efficient blending of landscape textures which isn’t really available as a shader.