I don't like that much IPD design.

From what the specs say, it is essentially a PPM system with some logic to filter out interferences. So the servo position is encoded with an ANALOG modulation.

PCM on the opposite is a DIGITAL modulation, which offer far superior signal robustness: it is very hard to corrupt the signal, and if that ever happens the receiver can know it thanks to a checksum attached to each frame - to me, the two are not even comparable!