I agree that it’s dependent on the individual horse’s needs. My horse has never really been ridden in an arena as at my farm where he was raised, started, and trained, I just rode in one of the pastures. So, a 2 acre field was the closest thing he experienced to an “arena.” I showed him and he was fine in arenas, including indoors. I sold the farm and the boarding barn I moved to had an outdoor dressage ring and a covered dressage arena. He disliked the covered (footing was deep) and was mildly more tolerant of the outdoor. He preferred, as expected, to be worked in a big, open field (which luckily we had). He also preferred being hacked around the property at all three gaits to endless circles and schooling. He eventually began to realize the “work” happened in the big field by the covered arena, and he would go from perky and forward to sluggish and Eeyore-like as we reached the field.
At our current barn, I try to sneak “work” in without him realizing it. We always start with a walking hack on a loose rein through the woods and around the property. If he’s feeling particularly sluggish, I’ll take him somewhere we don’t normally go (we can ride out into the neighborhood if we want) so he gets on his toes and is more forward. I use that energy to get him going and eventually we wind up back at the big field (this barn has a HUGE open field) where we work a little and I get him moving well. Then we’ll head off around the property at trot and canter and back through the woods before coming back down to a walk and finishing with a nice walk around the property.
There is a small outdoor ring at our current barn. I’ve rarely ridden in it. He doesn’t see the point, LOL. He’ll be a good boy and all, but it’s obvious he’s none too thrilled with going round and round.