Bad experiences with treeless saddles are usually due to either a poor quality saddle, a saddle not suited to the horse's conformation, an inadequate saddle pad for the horse and/or rider, or a combination of the three.
Both my husband and I are well over 200 lbs and we have no problem riding our horses in my treeless saddles, for 45 minute arena rides (moderate to heavy flat-work/Dressage training) to 4-5 hour trail rides, up and down hills at varrying speeds. My horses have healthy backs and a good work ethic. My chiro, who is also a liscensed vet and does accupuncture, is thrilled with my horses' condition, even though she has also seen train wrecks with other treeless saddles. My horses have helped convince her that treeless is not necessarily bad, just not a "one-size-fits-all" solution like many saddle makers try to convince you of.
And with treeless saddles, your pad is just as important as your saddle. Some treeless saddles can be ridden without a special pad, but only by a lightweight riders who are also light in the saddle (steady, not bouncy). MOST riders using treeless saddles need to use a quality, shock absorbing pad with spinal clearance. Since I am a heavy rider, I used a custom Skito pad with a ThinLine pad underneath. This pad combo set me back around $310, just for the pads.
Going treeless "right" is not cheap, nor is it always easy. However, once you find the perfect saddle and pad combination for you and your horse, it's heaven...
I would recommend to anyone interested in treeless saddles to join the group I mentioned, post your stats (height/weight/saddle & preferences), your horse's stats, pictures of your horse, and the type of riding you do on your horse. Members with similar horses or body types can give you their experiences, and help you narrow down your choices. Once you have one or more you want to try, contact a reputable dealer and have a demo saddle and pad sent to you, or see if any of the group members live near you and will let you try out their saddles (I have had a few people come out and try mine).