Well..
No one really uses them much, so I'm not certain what rules need be in place..
You can shift to and from it easily.
You can take large groups of people or things like buildings in with a miracle.
You decide how the laws of physics work in it..
You can use it to move from place to place rapidly..
“She'd become a governess. It was one of the few jobs a known lady could do. And she'd taken to it well. She'd sworn that if she did indeed ever find herself dancing on rooftops with chimney sweeps she'd beat herself to death with her own umbrella.”
― Hogfather