Mainly OpenSCAD is not a BRep modeling tool! It is not on the same level of power as CAD tools with a BRep kernel and this especially shows when you want to do a fillet over an arbitrary edge. Unfortunately these kernels are hard to make and integrate and I only know of two open-source BRep kernels out there: OpenCASCADE (used by FreeCAD and build123) and truck (not sure what the status of it is).