As we know, with iOS 7, Apple Inc. skipped the possibility of evaluating NIC (as a unique Devices identification) with any iOS-Devices.
The current work around by FMI presumably is built on a unique string within the FMGo applications path. Though same FM solution will show different PersistentID's on same device for different FMGo versions. Also, deleting and re-installing FMGo may alter the PersistentID.
A persistent PersistentID should be built on a Devices' ID as the Serial-# would be.
If Apple Inc. does not allow to evaluate the Serial-# of a iPhone OR iPad, this should be brought forward to Apple for reconsideration.