Just try to figure out why some owners have the PTU problem, and others don't.
For those owners who had PTU problem, did your Edge work in 4WD mode for a long time? Or in other words, did you drive in bad road condition for a long time, did you have heavy foot on the gas?
I just heard some explanation for the AWD system from a mechanical engineer who works for a auto maker (not ford). He said most AWD's center lock is not mechanical lock, and the torque transferred through PTU will cause heat. Working in the 4WD mode for a long will eventually cause over-heating, and damage the seal of the PTU. Most PTU has this potential problem. The only solution is not to force the car in the 4WD mode for a long time, so PTU can cool down.
Sounds make sense to me.