You can get away with exterior day for night, but you either need the right weather (a day shot will always have a sky that's brighter than the action on the ground, so you need to be able to counter-act/avoid/accommodate for that, and pretty much any daylight shadows will look ridiculous) or tons of cash to be able to run giant silks, singles, doubles, etc to plan for the scene in advance, as opposed to just trying to use it as a post-production catch-all.