UI/UX is not the difficult part. The hard part is the geometric modeling kernel.
If you've ever done UI/UX research and worked with volunteer developers who only care about technical problems? It is the hard part. Good UI/UX is hard to begin with, its even harder when no one is interested in front-end development.
In practice everybody uses an off the shelf modeling kernel like Parasolid, ACIS, C3D, or OpenCascade.
The history of FreeCAD proves that UI/UX is the hard part.