From what I can understand, the Geometric Modelling Kernel, commonly shortened to the CAD Kernel, is the part of software actually responsible for creating the mathematical representation of the part. In FreeCAD, I heard a few problems were attributed to OCCT, and Fornjot seemed to be making their own, so I set out to research further into potential kernels over the past 3 days. 

• Open Cascade is the most fully featured, it seems
  • https://news.ycombinator.com/item?id=24516438
• BRL-CAD Kernel can't do fillets apparently
  • https://forum.freecadweb.org/viewtopic.php?style=4&t=42009