All of the chain drive tillers i've seen, have the chain in a case and no dirt or dust gets to it.
Over the years we have had two Troy Horse model tillers. Both had had belt ajustment problems, leaked out of the axle because of bad seals and other problems. Now that Troy is owned by MTD, you know that they are going down hill!!
"If" i was starting out again, i'd buy the BCS tiller, it's built so much better than a Troy it isn't even funny!! The BCS has a "real" clutch, and NO belts! Also, you can buy the BCS with the industrial Honda OHV motor!