Single wheel trailers have a clear advantage on rough terrain since the trailer's wheel will follow almost the same line as the wheels of the bike. With a two wheel trailer the trailer wheels are likely to hit far more obstacles. I was towing my two wheel trailer through the streets of Detroit in the wee hours of the morning today on my way to the airport to catch an early flight home and certainly noticed that the trailer hit quite a few potholes that I hadn't seen or had avoided with the bike wheels. But being on reasonable pavement it wasn't an issue since none of the holes were bad enough to pose a threat to the tires - the situation would have been different in an off-road trip.

My trailer attaches to the chainstay near the dropout and I don't see any problems with that attachment point. It also lets me carry anything I may want on top of the rear rack in addition to towing the trailer which would be a problem with a seatpost trailer mount.