They need to be fitted, just as a treed saddle does.
Without a tree you dont have anything to help spread the pressure and riders weight. So for novice riders this can cause problems. It can also cause pressure under the stirrup bars or riders seat bones. Obviously a badly fitting tree can cause pressure problems too but if it fits then it will spread the pressure better- which is better for the horse.
Ive found many treeless saddles to have the stirrup bars too far back and hilarious to jump in!
Treeless have their place but they are not the easy answer to a horse that changes shape that people often think they are!
I have 2 Heather Moffett part treed saddles and I thought they were the answer to everything until I bought another horse. They fitted my connemara perfectly and he goes like a dream in them but did not suit or fit my new horse at all. I think they suit some horses but not others. I finally bought a balance saddle for my new Section D and he loves it but I hated riding him in the HM. I love the principle of the treeless-part treed but they do not suit all.
Had a chat with my saddler today who was out to refit the boy's treed saddle (to an XW -he's put on tons of muscle, apparently and was declared 'not fat' by both vet and saddler)
I mentioned treeless because of the issues fitting a decent treed saddle to a round cob and the slippage. The saddler said (just his opinion, obviously) that treeless are great for the first 3 months then cause pain because of the lack of distribution of weight and are therefore not something he would dream of recommending.
I have no experience of them and as someone who is not skinny, I would not want to subject my horse to additional strain therefore am not going down that route.