You can't spy productively on XC nodes because private transactions are fragmented and because every node forwards them trustless. This creates a scenario where there's no telling if the sender/recipient of a fragment is the original sender or final recipient, and there's no telling whether the amount sent is the whole amount.

Furthermore since fragments can be sent, even non-private transactions become "private" in this respect, since the amount sent could be a fragment, and so a spy has no grounds to assert that the amount sent/received is the total amount, or that the sender/recipient is the original sender/final recipient.