Trust a horse used to that country? Yes. Trust ANY horse in rough terrain? No thanks. I've seen Mia sprint unharmed across a field of 12-24" sharp boulders, but I've also watched her fall riderless in our arena - dry, level footing. At a canter, she used to stumble every few minutes...although she is getting much better and no longer tries to canter with her nose 3 inches off the ground.
Even Trooper, who grew up on a ranch in central Utah, has fallen chasing another horse (& while riderless). OTOH, I would trust our little BLM mustang pony. He takes care of himself at all times. I can't imagine him doing something stupid in the desert.
I've written before that someday I hope to tell people, "Mia is just a trail horse", and then secretly laugh because I'll be the only one who will know how hard it has been to turn her into one. Going down a small but steep grade last week, she finally tucked her hind legs under and did it at a slow walk...controlled. She was a PITA the last 15 minutes of our ride yesterday, but did a half dozen laps of cantering when we got back and stayed balanced & controlled the whole time. She hasn't stumbled at a canter once this week, which is a record for her.
It seems to me a good trail horse should be prized precisely because it is NOT something all horses are born knowing how to do. A horse who has spent almost all of her life in a corral doesn't know the same things as one that has lived loose. Lots of arena horses would need to learn how to move in rough terrain they have never seen before. And frankly, not all horses have the personality to be instant trail horses. The difference between Mia and Trooper is obvious in the picture below:
Yesterday, following Mia (who was on high alert for some unknown reason), my youngest shouted out, "Hey Dad! Have you considered medical marijuana? Maybe I can talk Trooper into sharing some of his stash!
I'm lucky. I have ATV trails starting 1/4 mile from here. If I needed to trailer Mia somewhere for trail riding, she would be arena only for the rest of her life. It would simply take too much work to make her a trail horse. The lady who broke her says I'm about the only owner she knows stubborn enough to try - although she agrees it is good for Mia. But not all horses are born ready for the trail...and some folks have no desire to force them.