Ive read a few SM novels and Ive just started on the "Gaunt" ones. What I keep asking myself is why there seems to be so little in the way of orbital bombardment. In the opening pages of the founding a WW1 situation is happening and has been for six months. The Guard are in their trenches and the traitors are in theirs, the planet (a forgeworld) seems to be completely wrecked with all the factories empty burnt out shells. Why don't they just Nuke the place from orbit? A sure way to break a strong defense is the limited use of "Tactical Nukes". Now I know the SM's have some kind of virus bomb that can kill an entire planet, but does the Imperial Navy have something similar? Are nuclear weapons available in the 40K universe?
In the Space Marine novels there were quite a few times, especially when Nids where involved when I thought "Why don't they just kill the Planet?, its lost anyway".