My car (2011 Ecoboost Flex) has 170k on it. The PTU outlasted both the engine and transmission and still going strong. My use of the car isn't abusive, but I am not exactly nice to it either. I've done a few drain and fills on it since I've owned it, about once a year. These D3 cars had a good belly pan design that directs fresh air over the PTU, so the cooling on it is adequate for most applications as long as the oil is changed frequently. Some models like the police cars and 13+ PP SHO's have an aux cooler. If you're going to track the car or tow a lot with it you might want to get one with the PTU cooler. Otherwise, as stated above, the standard drain and fill seems to make these reliable.
Edit: changed CD4 to D3 above. I've been playing around with my Mazda 6 lately and I used to own a Fusion so I had my wires crossed. Sorry for the confusion.