The reason you do the crash in miniature and supplement it with CG is physics - you'd have to pay for multiple debris passes, tons of fragments...doing it with a model means all that stuff happens naturally.
I'm reminded of Favreau on
Zathura - he used men in suits for the creatures. There's a shot where one of them skids into a table, knocks a ton of stuff off of it, then rights himself and keeps running. He said that that was the benefit of practical effects: it was an unscripted but cool moment that, if they had tried to do it with effects, would have meant figuring out the physics of the slide, the physics of the moved table, the physics of everything that fell off the table, the subsequent impacts and bounces and rolls - every pass at which cost time and MONEY. In the end, all they had to do with CG was a head removal.
In the
Serenity sequence, the supplemental CG was much cheaper than it would have been to create the envioronment, the ship, the explosion, the debris, the physics - each pass is more money.
Meanwhile...this:
Serenity (2005) | Grant McCune Design