Most of the problems with booleans in Carrara are due to bugs in the vertex primitive and really cannot be overcome... you can work around them, for instance there was a specific bug where if you placed a cube and a sphere at the origin then did a non-uniform scale on the cube(there was a specific range but I don't remember what), did a subtract of the sphere from the cube, it would crash. But only if the fidelity was cranked to %200. Set the fidelity to %199 and it didn't crash. As for the old flying polygons, that's a different issue. The first thing to do is a test render on the area to see if they're actually there and not just being drawn by the interactive renderer. If you see them in the vertex modeler, they're there. Undos and booleans don't mix because half of the time information doesn't get written into the undo buffer properly, if at all, causing many a crash on undo. I think the key is high polygon count and medium fidelity. But there's no real cure.