Rear fender lights are vulnerable to damage, sooner or later you will smack it against a door post or bike rack.
Rear rack bracket is a much better location, out of the way but visible. I lash stuff to my rack with a lamp no problem at all. You have to take care with the cable routing, some racks have hollow tube access for running cable. I run from the seat cluster, to the rear fender, protected by tape, then along the top of the fender, then up to the lamp. Use glue and ductape.