@Ceciliabr

It's like shininess is either on or off, with nothing in between.

In the Modify furniture panel, [font=courier new]Matt=0[/font] and [font=courier new]Shiny=0.5[/font].
In the Furniture materials panel, [font=courier new]Matt=0[/font] and [font=courier new]Shiny=1[/font]. The Java 3D preview lets you see the difference in shininess.

When rendering, Sunflow applies a shader to the material that "changes" the shininess. (see pages 14 to 23 of Sunflow Wiki PDF)

Is this just a result of the limitations imposed by SH3D, or is this a real Sunflow limitation?

As Puybaret had told me : "There's no limitation".
Sunflow's standard configuration in Sweet Home 3D is based on several tests performed in 2010 (Test 1, Test 2).

Enko