While this is immensely cool and very well done, it is worth noting that it's not exactly low budget.
Unless they had all that hardware sitting around ahead of time, the cost of creating the model shots is almost certainly > the cost of hiring an artist to produce what are really only a handful of exterior shots.
That said, I really like the look of this production. It absolutely wouldn't have had the same feel with CG, and I'm sure it cut down on post-production to have everything done in camera.
The bottom line, I still think CG would have been the less
expensive option.. But I'm glad they did it the way they did it