Not sure how this plugin works. Does it create a spline or does it define its own ‘arrow’ object?
One possible solution would be to lower the phong shading angle on the Extrude NURBs phong tag some, but I think that would only be hiding some of the geometry issues. Another guess is that the Fillet may be misbehaving, try changing the settings on the Extrude NURBs Caps and see what you get.
Darby Edelen