The problem is in game editor lot of times you need to adjust terrain height, repaint areas like paint grass ,ground texture after placing some house or structure : something you can't do with an external terrain editor.
So for me Shiva is not the tool for advanced terrain games.
Indeed if terrain is jsut in background or for some 2.5D game or a simple terrain to shoot ennemies it can be ok !
But for precise levels like advanced village , shiva terrain system is not the best or you could loose lot of time in workflow compared ot real time editing.