I have issues with custom collision meshes when they have curved surfaces.
I have a mesh that encompasses the four sides of a house with a door and 6 windows with rounded top parts. The mesh is quite simple, except for the rounded window tops. I then copy this mesh and rename it to act as a custom collision mesh.
After import into UE, I get the correct mesh, but a distorted collision mesh. it seems as if some vertices of the collision mesh move up Z for some reason. Also, the bounding box is three times a high as the one of the actual mesh.