Why are fillets so bad?

[ftzuk]

Another instance where Rhino fails but other cad software manage to fillet:

Strange Fillet Edge Behaviour

Can anyone explain why applying FilletEdge Blend Edge or Chamfer edge doesn’t work on the attached ? FiletEdge.3dm (193.8 KB) here’s the curves [image] here’s the solid [image] here’s the result of filletedge [image]