I build my own wheel autopilot
drives and use a toothed auto timing belt as the final drive. The motor
end sprocket end is only about 1 1/2" in diameter and whilst the belt does not work hard it is constantly being flexed around this small radius and should suffer a lot of fatigue. The last one lasted 8 years of fairly constant use and finally gave up more from UV damage than anything else. These belts are designed for in excess of 100,000km of use before replacement in auto engine
Based on this experience I would offer the opinion that either these synthetic cord belts or the stuff they use for making slings would probably achieve better fatigue lives than many of the cable systems now in use.