Other AFV engines have been mentioned in this thread by several posters inc Fox . Regarding B Range engines, they can be prone to blowing head gaskets between certain pairs of cylinders and heads often found to be in need of facing before refitting due to bowing, so this would be one reason for changing a unit in the field as surface grinding facilities probably not available. Also they can suffer valve burning.
When you change an engine, you generally only indent for one item, but if trying to repair an engine in the field, a number of items would be demanded, only wants one critical part to be n/a and the job stops dead.