I have a question from the opposite perspective. I have never used Phantom mode. I researched NAZA mode before I bought my Phantom because those "in the know" said that was the way to fly, and my initial setup was in NAZA mode. Therefore, I really don't know what happens when things go wrong in Phantom mode.
In NAZA mode, if my satellites drop to 5 or less, I have the option to switch to ATTI mode; it's not as stable as GPS mode, but it's definitely more stable than maneuvering my little Hubsan X4. What happens if you lose adequate satellite contact in Phantom mode? It seems to me that not only do you lose GPS stabilization, but you also lose RTH capacity. Does it revert to pure Manual mode?