> Fillets and chamfers are a good example. They seem simple but are geometrically non-trivial, and OCC will fail on cases that Parasolid handles without complaint.

A long time ago I interviewed at one of the large CAD companies. I remember getting an office tour and the person showing me around pointed into a corner with six desks and said "that is the team that does fillets".

Open source tools can handle some cases, but to handle the full complexity of real world problems is a huge extra step that I doubt they will manage any time soon.