Rather a vague question. The design codes may make conservative assumptions in some cases. And the design codes may neglect localized stresses around nozzles, at changes in thickness or shape, etc. The design codes may be considering buckling that doesn't show up in a FEA. And who knows, maybe the FEM method has been done wrong. Anyway, we can't very well design everything in the world with FEM, and if there were easy and simple methods that gave identical results to FEM, there wouldn't be any reason for FEM to exist.
