Many people don´t get on with a treeless saddle, first and foremost because they don´t purchase the correct pad to go with it. Secondly, and sadly still all to common, because they don´t have a balanced seat and find they either tip forward or backward in a treeless down to their own error, or feel "unsafe". None of these problems being the fault of a treeless saddle. I have friends who jump in treeless saddles, and do so very successfully, as well as plenty who use them for trail riding and some who use them in dressage.
I personally have tried three different brands of treeless. My main discovery is, if you buy a cheap one, well, you get what you pay for. But that is pretty much true of any saddle. When properly placed, with the correct pad, they are great for trekking. Personally I would not use them for great distances and speeds. Remember, this is just my opinion, but I think the spine needs some relief if you are constantly polling along at 25 to 30 kph. They work great on most horses, most horses being triangular in shape. But on rectangular horses I found myself either on the neck or on the haunches at every decline or incline. Though rectangular horses are few and far between I seem to have 2 out of my herd total of 14!
Like bitless bridles, there are now many different treeless saddles to choose from, so if at first one doesn´t seem suited to your horse, go and try another. Also, like bitless bridles, not all are good. Buying cheap as far as a saddle is concerned is a false economy. The money saved will be spent on a chiro, osteo or massage therapist for your horse. Nearly all my saddles have cost more than the horse, even second hand ones - and there is nothing wrong with a well cared for second hand saddle. If it´s leather it will save you a lot of elbow grease getting it supple. Someone else will have done the work for you ;)