derailer in the spokes on standard bicycle is usually caused when chain gets caught up and pressure on peddals pulls derailer into wheel. would usually be in top gear, closest to spokes. a cause of failure with motorized set up as i could see is such high revolutions of chain could make it hard for jocky wheels in derailer to keep up, any drag or friction would again pull derailer to its doom in a mess of broken spokes. maybe if this high revolution aspect was avoided by adjusting riding style, ie avoiding reving the guts out in low gears, problems may be avoided? shimano make a cheeper rear derailer with extra large jocky wheels this could help?