There's nothing magic about PPM. The radio looks at all your controls, and generates PWM lengths for each channel. Then it combines them into a single PPMSum stream, and transmit it. The Rx receives the PPMSum stream, and breaks it up into channels, and sends it out to each servo channel port. Then you would use 8 wires, and send them all to your flight controller. Back in the day, the APM had a companion processor, that would take the 8 channel inputs, and combine them back into a single PPMSum stream that would get sent to the main processor, which would then read them in as 8 inputs again.... It's a LOT of back and forth. In the case of Pixhawk, you'd have to use an 8 Channel PPM Encoder, to externally combine the 8 channels back into a PPMSum stream to send them into the main processor.
Using PPM Sum just makes sense. It is actually so much simpler. Your Tx reads the controls, determines the PPM lengths, combines them into a PPMSum, transmits it to the Rx. The Rx takes the PPMSum and simply passes it through to the Pixhawk. The Pixhawk decodes it back into channels.
Every time you combine or disect the PPMSum signal, you introduce errors which can cause jitter and lag and... It's really anachronistic to send 8 channels into a flight controller.
The *only* advantage it 8 channels is that theoretically, if one of the wires has a problem, you still have control on the other 7. With PPMSum, if you lose the single wire, you lose all control. But this isn't really a problem. Losing one channel out of 8 means you have lots partial control. You can't fly a copter with the throttle channel, or the pitch or roll or yaw or mode channel not working. And now you have 8 possible failure points, instead of just 1. With a modern flight controller like Pixhawk, if you lose your PPMSum wire, it will go into failsafe, and do what you program it to do, which is usually an RTL. If you lose a single channel when using 8 wires, you'd probably have to switch it to RTL manually, but what if the channel you lost was your flight control channel?
Regardless, after flying these things for 3 years, I have yet to have a signal wire failure. I've had lots of failures of various types, but never a signal wire.
