Thanks, that's very interesting. Looks like the Futaba S-Bus is just a RS232 connection: https://developer.mbed.org/users/Digixx/notebook/futaba-s-bus-controlled-by-mbed/ At least it has a parity bit, but would be better to have a checksum. I guess in a plane with big motors there is a lot of noise. Two wrong bits and bad things could happen, especially with the digital channels.
The probability of errors is very low (if you've got enough noise to disturb that you would also had disturbed the old PWM signal), and should one byte be corrupted it's going to be corrected 9ms later which is pretty much faster than anything mechanical behind can react anyway.
Big motors historically would cause interference on the RF side, not on the demodulated signal, but that's a thing that's been buried and forgotten since we have 2.4GHz digital RF.
Nowadays the main source of problems is actually power supply issues rather than signal problems, mostly because of the lack of awareness from people about how it works and how important it is and bad quality / lack of characterization of the components.
The thing is that when you build any standard setup there's pretty much always one of the elements that will have one of those "BEC circuits" (voltage regulators) built in so that things just work as soon as you plug everything together on the ground... but when you ask most people won't be able to tell you how or why it does, probably won't even know there's a voltage regulator somewhere, why it's there and what its limitations are so it could well not be up to the job once things start moving. It's only when nothing powers up because there is none that people start asking questions, and on the web they'll be quickly redirected by people who know "you need a BEC" but don't understand the parameters either to whatever crappy $3 Chinese regulator that may be noisy as hell and might or might not suit the setup's requirements (as someone already mentioned, what Dave describes as a "cheap voltage regulator" is actually among the most expensive ones you'll find on the market that don't have other functions, and it's comparatively very well characterized, usually on cheap units you only get no-load voltage and an overrated max current and just have to hope for the best for the rest).
DJI stuff is excellent for its plug and play aspects, awesome capabilities and seamless integration, but scores below zero for the DIY interest. Closed and proprietary, the whole system is extremely complex and made of so many components it's virtually impossible to figure out anything and each product is significally different technically so any findings you could do would be obsolete 6 months later.