There is a very simple reason for why Πίστις Χριστου is not an ambiguous construction. It is also the very same reason why the meaning of the construction in encoded by the grammar of the language. This is also why I lean toward an objective genitive view. And its simple too.We, non-native Greek speakers (or I should say “readers”), believe its ambiguous because it looks ambiguous to us.
If the construction is truly ambiguous to the native speaker, why in the world do none of the Greek fathers read the phrase as being a subjective genitive? I would suggest that perhaps it isn’t really ambiguous to them.
See especially: Roy A. Harrisville III, “Πίστις Χριστοῦ: Witness of the Fathers,” Novum Testamentum 36 (1994):233-41.
Other good points include Loren Rosson’s The Meaning of Pistis Christou