I have yet to find a trailer or a horsebox that seems to take into account the safety of the horse inside it or the fact that the vast majority are driven and used by women. Take the Equitrek for example, the bar is fixed so that in an emergency it would be a hell of a job to get it down with a horse that is panicking or worse still has gone over it. There is only one way out of an Equitrek for a horse as the back door is narrow and only intended for humans. The partition is so heavy that it takes two determined women to shift it around and a lot of swearing and unladylike grunting too. The outside doors are flimsy and sharp around the edges so that if the horse tests them with his nose he can get hurt. The latest Ifor Williams, functional as always, still has sharp metal bits at the top of the central pole and the catch on the inside of the jockey door would be a real hindrance if you needed to get out in an emergency. These days you can have the front ramp on whichever side of the vehicle you like but for those that are on the right, you may be unloading a horse onto the traffic side of the vehicle in an emergency. There are no brackets on the back of the trailer nowadays either so where someone wanted to use panels to enclose the horse for protection or training, or in an emergency situation, there is nothing to fasten them to really easily. As with the Equitrek, the front bars on these trailer, and most others, are so thin that they must cause bruising if a horse pushes against them due to heavy braking or poor driving. I think is a common cause for horses not wanting to load - many seem to go in easy if the front bar is down whilst loading. In horseboxes their seem to be plenty of places for the horse to bash it's head in the underside of partitions or sharp edges to "luggage racks" or to hook up a headcollar on catches and brackets.
Apart from the physical effort involved in hitching trailers up and moving their contents around, it could be argued that they are fine for the well trained, calm and experienced horses but for the early stages of training and especially remedial training, they are cumbersome and sometimes dangerous.