Grahame I'm not sure but one factor is the minimum power level that the flash can put out (ie the minimum duration of the flash). Flash exposure depends on flash power, flash light path, distance, aperture and ISO. If power, aperture and ISO are all at their minimum (highest f stop), then there must come a point as the distance gets smaller where over-exposure occurs. This is alluded to in
this Nikon article. It depends of course on whether you are using direct flash or not too.
Another factor could be the largest beam angle of the flash. If the flash is too close, you could get uneven illumination across the frame. Speedlights usually have a minimum focal length setting.
Dave