Why can't any of the transparency modes be perfect?

See the documentation on SoGLRenderAction::TransparencyType, at http://doc.coin3d.org/Coin/classSoGLRenderAction.html

For Coin 2.3, a new transparency mode called SORTED_LAYERS_BLEND has been implemented for SoGLRenderAction which uses a technique that actually produces correct transparency rendering.

For earlier versions of Coin, transparency rendering will often fail and produce visual artifacts because blending is done in an incorrect order. To avoid visual artifacts, all polygons must be rendered in sorted back-to-front order, and intersecting polygons must be clipped against each other. This is in general too expensive an operation to use in a real-time system, so it was not implemented.

Note that there is a transparency mode called "SCREEN_DOOR" that does not rely on blending - it uses dithering - that has been in Coin since v1.0. This transparency mode always produces correct results, it doesn't even require multipass rendering, but since the visual result looks somewhat dated people seem to prefer the blending techniques.