I will cautiously weigh in on this topic. I show CDE, however I started out in the breed ring. The main problem I personally (this is just a personal opinion) had with show ring driving is the equipment "required" to show. I don't and wont ever use checks/ martingales on my horses, as I have two that required a lot of re-training from previously being made into "pleasure driving minis". I personally dont understand the logic of using two opposing forces (martingale and check especially in the pleasure classes) to get a horse properly into a proper driving frame as outlined in every carriage driving book out there. In every book I have ever read its a big no-no. When I watch breed ring classes all I see are a lof of very uncomfortable horses who are mostly counter flexed.
This is mainly why I can't cross over into the breed ring, because I refuse to show with a check (which is required by AMHR rules) it doesn't matter that my horse can keep his head in a naturally high head set without a check. However when I drive him he is free to put his head wherever he chooses. He uses himself properly, has got a great extension and we excel at CDE. We both love the challenge.
I don't feel that I would cross over because of a couple of things:
If AMHR dropped the check rule, maybe I would consider trying again, but I really don't see that happening.
I got bored, and nothing beats being out in the outdoors doing so much more than driving in circles.
A couple of points about CDE
1. No you do not need solid tyred vehicles until you move past training level, but thats the beauty, if you dont want to go prelim then don't, every event can have a different dressage, a different cones and obstacle in the training level.
2. You do not need to canter at CDE's I have heard this from many breed ring drivers "well I dont want my show horse learning to canter because he will do it in the ring". Cantering was not even allowed in the hazards in training level until very recently and it is optional for training level.
In the end however, I will come across to those that show in the breed ring as ignorant because of my views on checks and martingales, however I am fine with this. It has never been my intention to say that people should never show in the breed ring, but I think show ring driving has gone in the wrong direction for a long time.