I have just tested two DXFs created with "Drafix Windows CAD 2.1a: October 26, 1992". QCAD Professional (trial) (ie. Teigha) opens them mostly fine (there are accented national characters in texts, those are incorrect, then again, non-ASCII stuff in DXF tends not to be portable anyway, so this is more or less expected).
QCAD Community (ie. dxflib) doesn't open them fine -- things are mostly there, but all texts are stacked on top of each other instead of being in their proper places.
Unfortunately, I am not in the position to share the original .cad or the converted .dxf files (but I can likely create some samples if Andrew wishes to investigate the dxflib issue).
Its "Ultra Drawing (*.DWG)" export format does not appear to be AutoCAD DWG in any way, shape or form.
Hope this helps.
Ed.: The drawings tested are not particularly complex, rather the electric diagram type, but their dxf representation still has layers, blocks and various element types that all seem to be OK.