>> How does Dune3D compare to FreeCAD?

Dune3D is more like Solvespace with a few improvements and bug fixes vs being anywhere near FreeCAD in terms of capability. Improvements include using STEP files in assemblies and having some ability to make Fillets or Chamfers. Bugs fixes would be due to using OCCT for NURBS surfaces - solvespace frequently fails with NURBS boolean operations.

As for overall capability, FreeCAD does everything these others do but also supports lofting and other modeling options, BIM for architecture, I think it does pre- and post- processing for FEA, and maybe some other "big tool" things.