I would say that a large proportion of class 1 assessments probably fail on reversing. You can expect something more challenging than the DVSA test reverse, but I have no specific information about Fedex. Possibly putting it on a bay with a trailer either side or something along those lines.
You can only go and try your best and if not successful, then you have had the experience.
I failed an assessment straight off test and then I was back at the same place within 3 months having gained some experience on agency shifts.