Jadasc raised a great point here.
There are few problems that can challenge a mage that won't completely outclass a vampire, unless the vampires are extraordinarily experienced or well connected and the mages are barely out of apprenticeship.
Add to this changelings and werewolves and who knows what else from the World of Darkness game, and it can lead to a very unbalanced set of characters.
At what point do each of the 'races' become balanced with respect to each others' abilities, if at all?
Is such a group possible?
Edit: An example from a comment I made:
If I am playing a Vampire/Promethean game, should the Vampires start at two blood points? Should they be given some XP to level the playing field? Or should the Promethean players be handi-capped? Is it a case of working the story and challenges within so both 'races' can contribute equally?