They do have to "fit" the horse. They are NOT a "cure all" and fit everything, like some people like to claim. The front pommel needs to match the angle of the horse's shoulders.
I have a Bob Marshall endurance saddle and LOVE it! I have friends with the barrel saddles and they really love them too. They're quite secure, though there is a little "play" in the side-to-side motion.
They will keep their shape and will be very comfortable for horse and rider IF you use the right pads with them. The most recommended pad is the Skito Treeless Saddle pad
. They are custom made to order, based on your weight, your horse's conformation, and your saddle type. I have a Skito that I use with my BMSS and my horses are never sore, even after multi-day rides (and I am a 250+ lb rider!). Supracor
, and Saddleright
are three other brands that are also used with treeless saddles.
You need to choose a pad that gives spinal clearance and distributes your weight, since there is nothing in the saddle that does those things.
To increase stability, I use a roper-style 100% mohair cinch. It works like a charm!!