For Sheet Metal I mean you cannot make flat patterns of parts which account for bend deduction etc.
For Weldments you cannot use profiles and make cut and weld lists.
For plastic part design you can design parts in any program if you’re already an expert (though it would be slower and more painful in Rhino over a proper CAD program and from the OP’s post I am sure he is not) but using the dedicated plastic design tools and the injection moulding simulation in SolidWorks or Fusion or Creo etc. are very useful.
I am talking design. Design for Manufacture, for actually getting things manufactured to real world limitations and not just making pretty 3-D geometry. I teach Rhino at University level (College in US terms) but it is not a proper CAD program for anyone that knows a mechanical/parametric modeller.