Recommendation for a failsafe
For slide carbs, a good throttle return spring mounted on the servo arm AND a rubber band around the carb slide is the safest method.
An Electronic failsafe will NOT stop a runaway if your linkage pops off and tension from the rubber boot holds the slide open. (had it happen)
For Rotary carbs, a return spring alone should be sufficient on the carb throttle lever.
FYI:
Team Losi has a very good Throttle return spring kit for less than $5 usd and comes with two springs and brackets to a package.