*DANGER* - A seat post mount will transfer the weight of the trailer to the seat post when the brakes are applied. During a turn, that weight transfer will have a tendency to topple the bike since the attachment point is high up and the load is being applied sideways. Trailers mounted at the rear axel or chain stays do not have that problem to the same degree, since the weight transfer is much lower down on the bike. A lower mount limits the amount of leverage given to the sideways pushing force.