As Timb0h says, I would have thought that there is a very real risk that he could injure himself in a harness if something startled him when out. Bunnies have a very delicate skeletal system and damage to the spine can be caused relatively easily.
It also of course depends on the rabbit as to whether it will tolerate wearing a harness or not. Some are fine with it, others get very upset when it's put on x