Manual (page 7) says "Never use non-digital servos with HRS receivers. Severe damage to all electronics may result, including the possibility of a fire".

That advice is in a highlighted box to make sure you notice it.

This might be diff for FAAST, but I tend to doubt it as FASST is just a diff tx module and rx. I'll check the instructions that came with my FAAST module/rx to see if it says anything diff, I can't find those instructions online.