I've experienced signal loss a few times on my Phantom 4 at distance. It's very unnerving the first couple of times, but the RTH has always performed flawlessly. Generally signal is re-acquired really quickly and you resume the flight. Once or twice I let it come all the way back just for trial purposes and it worked exactly as it should.
I truly believe if you pre-flight correctly, the Phantoms are very, very reliable. If you don't pre-flight including all settings, IMU, compass, etc., a fly-away should probably be attributed to that.