I haven't seen anything about Transit transmission problems. They use the 6R80 which has been used on many light trucks for years, like the Ford F-150, including those with V8 and turbo V6s which have more power and torque. I'm surprised FedEx would be having problems unless they beat the heck out of vans with their constant stop and go driving.
I'm curious now if FedEx drives gas or diesel Transits. I haven't noticed any in my area.