To unload, you just lead him straight out. You are never trapped between a wall and the horse. You are never leading him into a dark, confined place head-first. He can see where he is going coming out, rather than backing out blind. You aren't driving him from behind in his blind zone.
Horses traveling facing backwards are able to settle in and assume a more relaxed stance.
Sudden braking is less of a problem with rear-facing trailers in terms of injury to the horse being less likely. If I had not stopped and freed her, but just gone on a few more hours, she might have struggled and panicked so much that I would have had a dead horse. These blood value changes all testify to the great physical effort required of the horse during conventional transport. Considerate driving is also key. Having one person riding in the trailer with the horse, in touch with the driver via cell phone, can be very instructive.