If you use Shell on a noisy surface, you'll get self penetrations. Even with Edit Normals, it will not correct it coz Shell uses face normals rather than vertex normals (I think).
This MCG will correct the Shell with a surface that is the same with the underlying geometry, you can do this manually with an Extrude using Group Normals rather than Local Normals. This MCG provides the convenience so you can change the Shell and not having to redo the Extrude.