The problems that can arise when looking to hire promo models come solely from when you go the risky route of hiring on your own instead of from an event staffing agency. When you hire promo staff
from job sites it is difficult if not close to impossible to verify that the person you are considering for the job actually has the experience they claim to have. You can verify job references by calling previous employers, but to really get an accurate view of how that person works in 3D, what they truly are trained and experienced in for this line of work, what their strengths and weaknesses are, you won't find out till day one on the job.
Another problem is that you don't have a guarantee that the people you hire on your own will show up on the day of your event. What if they take another job? What if they just forget or just don't feel like showing up? You could be stuck with a promotional event with no promo team.