It seems that cover and KFF needs the units to be entirely within, and since they are using cover as an example i have a feeling the KFF is working as the cover rule.
So units with muliple models gain the benefit of cover if every model in the unit is either on or within terrain.
So long as all the models in that unit are either on or partially within the terrain, they gain the benefit of cover.
The same for single models/characters, but maybe a vehicle like kans,dreads, nauts, battlewagon and the like needs to be at least 50% inside the bubble to gain the benefit?
Otherwise is seems to me that you still can make the bubble artificially bigger bij putting those models just a bit inside the bubble.
But it seems to me that GW just wants to get a away from that.
One of the reasons besides that units needs to be entirely within, is that the embarked KFF only gives the transport the save instead of creating a bubble centered around the hull.
You could cover the most part of your advancing army with 1 Big Mek with KFF inside a Battlewagon + putting models slightly inside the bubble or one boy to give the entire unit the benefit. (a bit Op really)