advantages are fitting, or more precisly, not fitting different horses. Seriously that is pretty much it.
they need special pads, they concentrate weight in one spot, they arnt as stable, basically a step backwards in saddle design. Ive seen the thermal imaging from even the good ones. They are not in the horse's best interest. Although lighter riders seem to be able to get away with them, the horses are doing ok despite the treeless saddle not because of.
There is nothign you can do treeless you cant do better with a properly fit tree.