I first tried treeless because I was having loads of problems finding a saddler who could provide me with a well-fitting treed saddle for my cob. I tried a friend's Bob Marshall western-style treeless and it felt as if my chunky cob had grown a hand and sprouted wings, he moved so differently and with so much freedom.
I then got one of the treeless saddles that has blocks for the pommel and cantle. At this point it's fair to point out that there is wide variation in style and quality in treeless saddles, and these with the blocks can cause huge problems if they don't fit horse and rider correctly. So my saddle was great for one of my horses, but not all of them.
Having tried rather a lot of treeless saddles, at the moment I now use a Sensation and a Freeform, and between them all 3 of my riding horses are sorted out. All of these saddles are used with an appropriate pad give clearance over the spine. I have adapted the pads by putting in "memory foam" (very expensive) and that I always ride with these under the saddle. This foam distributes weight, and I have tested this by having friends stand on my hands with the pads on top! So I assume it also helps my horses with my bony bum! So far I haven't had any pressure problems and I have a nice even wear pattern when the saddles are taken off.
At a clinic I ran last year we tested several treeless saddles using a Port Lewis impression pad and it did highlight pressure problems with certain saddles. The one with the blocks had pressure at the back - this was because the rider wasn't sitting correctly and was leaning back on the cantle. (You would also get this if the saddle was too small for the rider). The older-style Ansur had pressure on the sides of the wither, and we couldn't quite work that out, but it was probably because they are very flat, and the numnah recommended didn't add a gullet. A Torsion saddle showed horrendous pressure under the stirrup bars, and a slight line over the top of the back - well it seems the bars are hung off a single strip of probably webbing in the fabric of the saddle... and the rider was quite reliant on her stirrups. My saddles with their pads performed well in our amateur testing, with different riders.
Long, but I hope that was a bit interesting.