PTU (power transfer unit) is generally reliable, but definitely an area of concern and there have been failures. Not widespread at all though. I believe having the PTU fluid changed helps. I just had it done at my dealer. FORD claims it is lifetime fluid, which is nonsense. Some dealers will refuse to change the fluid based on that "lifetime fluid" claim.
Another good idea is to change the rear differential fluid (which all dealers WILL do). Tranny, rear diff, PTU - take care of them and hopefully they take care of you.