Why is it difficult to fight Astrum/Corsanctum? Astrum has 39 nobles, Corsanctum has 13 (total of 52). FR has 14, Asylon has 35, Niselur has 17 (total of 66). Granted, Asylon has a pretty lengthly refit line, but so does Corsanctum...
The same geographic features that make Niselur easier to defend, also make it harder to link up armies with. You are looking at a 48 hour trip to get through the mountains, and you have to have the food with your army as well.
