The Shell command currently uses the same basic algorithm as OffsetSrf with sharp corners. With complex objects, this usually fails to make a solid, and often leaves quite a mess… There are no options for this command other than thickness.
OffsetSrf with the round corner option is currently much more reliable, producing much more usable results requiring many fewer corrections. Therefore I think it would be good to offer the round-corner option in Shell as well.
Below a moderately complex object made with planar surfaces, the results of Shell and OffsetSrf/Sharp are pretty poor, while the result with OffsetSrf/Round is nearly perfect, leaving only two small naked edges which are easily fixed.
The OffsetSrfs are made after having deleted the bottom face, with flip to inside and Solid=Yes. In the images below, the backfaces are red, indicating open, unjoined surfaces.
ShellvsOffsetSrf.3dm (3.1 MB)