ArenaFrames, maybe since the sides are always balanced. BossFrames on the other hand would have a problem with people going into older raids solo or duo depending on the encounter. For example, if 2 people start an encounter that shows 5 BossFrames, only 2 frames would be created since the header is still looking at the size of the raid group instead the number of BossFrames needed for an encounter.
Another problem is that the buttons themselves aren't shuffled around. The header builds a sorting list from its configuration and reassigns the unit for each button according to its list. This means
UnitButton1 would be assigned the first in the list,
UnitButton2 would be the second on the list, and so on. This would end up overwriting your initial configuration immediately and every time the group composition changes. Even if this were handled with a
SecureHandlerAttributeTemplate watching the
unit attribute, sorting would be completely disabled and filter settings may truncate the list occasionally.
This info is from looking at
SecureGroupHeader_Update() and its local helper function
configureChildren().