Using stirrups with a pad with no tree can be dangerous. If you put too much more pressure in one stirrup over the other your bareback pad/soft saddle/treeless or even your treed saddle will roll. This is unpredictable and if knocked off balance badly your tendency will be to use the stirrups more aggressively to catch your balance, which may result in the pad rolling and you on the ground or worse having your foot stuck in your stirrup and drug by a terrified horse. My mare has high withers and a more prominent spine and my bareback pad with stirrups has never moved an inch much less rolled. But I cannot promise this for everyone.
There are a lot of different factors that affect the stability of any bareback pad on your horse. These are some of the factors that may affect the stability of your pad- your riding skill/habits, your horse's confirmation, the slipperiness of your horse's hair (like after using showsheen on the hair makes it very slippery) using a breast collar and it's location and fit, the type of liner you use between your horse and the pad, your ability to keep your seat not based on using stirrups for maintaining balance. My mare is unpredictable and my skills are not what they used to be so I know I'm taking my chances. That is my choice. You are responsible for your own choices. There is inherent danger when riding no matter what. Using stirrups with a bareback pad does raise your chance of having a wreck. Again, each of us has to make that choice for ourself. I love my soft saddle and have had zero issues with it. I do not mount from the ground but have many agile customers who can do this without their pad rolling.