Well digital encoding ensures that "glitches" corrupt the packet thats sent to the reciever. If the packet doesn't pass a validity check then it gets dropped and the servos don't get a position update from that packet. Enough failed packets and the system goes into failsafe.

Also, the systems operate well above the freqencies generated by rubbing metal and other common causes of 72mhz "interference" so while they are not "immune" they are much more "resistant"