In my experience, you should not mount a treeless from the ground. Almost all of the company websites will tell you this as well. Without the tree, they are just not as secure on the horse and you will feel the saddle slip under you as you try. I have tried a few different ones and each one, no matter the pad or how tightly it was cinched, slipped on my horse when she pulled a 180 degree spin. A fellow endurance rider in my region had to dismount on trail during a ride in a bob marshall treeless and ended up pulling her hamstring badly when she attempted to remount from the ground as the saddle pulled down.
Fit will also depend on the withers. Most (not all I believe) treeless saddles will not properly fit a horse with high withers - there is just not much clearance no matter the pad you use. Everything worth the try with the demo programs that are offered adn treeless saddles are fairly popular around the endurance rides I attend with the Bob Marshall being the most popular western style around here.