They can be if your baby is a scooter which most babies cant do that for quiet some time. The only risk is if the baby scoots over and presses his face up against the bumper. I used one with my son no problem, I think most people do use them. It is totally up to you but yes, they are considered a risk. I always placed my son in the middle of his crib and make sure the bumper was very tightly fastened to the crib. Noah didn't ever move from the spot I put him in so I wasn't worried. One thing that probably helped is that I used a sleep positioner for my son, he was a side sleeper so I needed to make sure he didn't roll on to his tummy. The positioner kept him from moving around.
You should however make sure there are not stuffed animals or blankets in the crib!!
I was told it was the plastic ones that caused more of a problem than anything. But i've been told so many things i don't know what to listen to anymore, every one i've ever known has used the cloth bumper pads and nothing happened. I agree with Grose, the sleep positioners are always a great idea ! :)
I used bumperpads with all my babies and everything they slept in and they are 7, 6, and 5 next month. I will use them with this baby. I wouldn't worry about them it's making sure the baby has nothing they can wrap there faces in or stuffed animals up by there head that you should worry about. You can always ask your pediatrician too. Good luck!