Coming from 25 years in the 3D visual effects and animation industry, I'm finding Composer's dumbed-down methodology to be quite the challenge in terms of creating correctly shaded and "real" looking objects. I know this is just a "quick and dirty" illustration program and not a visual effects package like Lightwave, Maya, or 3DS Max, etc.
One of those challenges is having only one light able to cast shadows. This flies in the face of traditional 3-point and visual effects practical lighting techniques where all lights cast shadows and the lighting director positions them to fill in and balance all lights together. Trying to "fill in" shadows with another light is acting no where near close to how the real world works.
Anyway, I know this is all based on the limitations of OpenGL and the very simplified nature of Composer's feature set and UI. However, there is one show stopping problem that forces me to not use Per-Pixel Lighting mode Shadows... it is shadows cast by a directional light "bleeding" from the lit side through to the unlit side of THIN plates. I've never seen this happen in OpenGL-based video games and wonder if others have seen this or know of a fix. Yes, I have a very capable and high-end video card in a top-of-the-line i7 4770 system and have set all program options to hardware accelerated settings and have set the shadow map size to max of 1024.