The prospect of losing critical staff in the disaster is often not even considered. It's not something any of us would ever like to see happen, and therefore people don't even like to think about it, but in DR thinking it has to be counted.
One of the DR scenarios in a place I used to work was a plane hitting the primary site during the afternoon shift changeover (we were near a major airport). All the operational staff were at the Primary, and the Secondary was dark. Therefore that would immediately take out 2/3 of the staff, plus most of the management layer. The DR Plan still had to work.