The core problem isn't memory safety, it's due to difficulties with the mathematical foundation of NURBS surfaces. For example, the intersection of two NURBS surfaces can't be exactly represented by a NURBS curve.
And then you export. Rather than exporting the entire procedural tree (which would require shipping the entire kernel), you approximate the curves with NURBS trimming curves. When you import from another CAD tool you need to deal with edges that only intersect within a given tolerance, and sometimes that tolerance is awful (Catia, I'm looking at you). Operations on toleranced edges are a huge source of complications.
And so on and so on. It would all be so much easier if the NURBS math could be exact
The simple explanation is the capability to model 3D shapes in a way that you can output the data to the rest of the manufacturing process. Hence what manifacturing process you target at least partially dictates your constraints.
The output could be - engineering drawings, CNC machines, 3D printers. Or another 3D modeling application down the pipeline.
In a way ”CAD kernel” alone is a worthless as a spec since it’s too general, just like ”a vehicle” is a useless spec for engineering.
CAD kernel- for modeling what, by whom, and where.
Naturally questions of numerical robustness and exactness of presentation soon enter the picture. How large or precise features do you need to model, for example.
Maybe SolidWorks handles it better.