Quite simple really.
I ported a building using Google Sketchup, got it into the game just fine. However when I added a collision model, it would always crash as soon as I loaded my map which I placed it in.
Then I tried to just simply make a box and give that a collision model. I placed it alone in my map and that also caused arma to crash on load.
Okay I made a box and gave it mass and made its collision model 5% smaller and that worked, but my large apartment building (5 stories at least) refuses to use its collision model.