The risk of a butt lift surgery, as with any surgery, is a (low) risk of post-operative infection, bleeding, and swelling.
Some risks and side effects include pain, scarring, wound dehiscence, allergic reactions, skin loss, and skin discoloration. There may also be fluid or blood accumulation, and asymmetry due to fat absorption. There is also a risk of the silicone implants slipping out of place and correcting this may involve further surgery.
Your surgeon will discuss all the risks with you before the procedure.