According to Moulin, there are several different sets of axioms that can be used to uniquely derive the Shapley Value, and Equal Impact is among them (it can be used to derive Shapley Value by itself, if I understand correctly).

The problem with all of those sets of axioms is that each set seems to include at least one axiom that isn't completely intuitive. For example, using the terminology in the Wikipedia article, we can use Symmetry, Additivity and Null Player, and while Symmetry and Null Player seem perfectly reasonable, I'm not so sure about Additivity.