Why do people complain about it giving horse back problems from sitting on it? It can't be worse than riding bareback and people do that all the time!
Depends. Most cross country, distance or competitive organizations prohibit the "treeless" just as they prohibit riding bareback. In spite of the treeless marketing that purports otherwise, treeless does not provide a structure to bridge the back.
Of your horse is young, or you are a normal sized person, do your horse a favor and lose the treeless. If you are a skinny and your horse is mature, enjoy.
If you are willing to spend for a saddle that is really kind to your horse, and is very comfortable for you, look at a flex panel.
If at first you don't succeed, figure out what is wrong, then FIX it!