The FDA and the Consumer Product Safety Commission have issued a warning that infant sleep positioners should not be used. They cite 12 known cases of suffocation over the past 13 years. Babies can become trapped between the positioned and the side of the crib or bassinet, or the mattress and the positioner. There are many more reports of parents finding infants in unsafe positions next to the positioner. More information about the warning can be found here.
Placing infants to sleep on their backs helps to prevent SIDS. Positioners, pillows, thick blankets, and other soft bedding materials increase the risk of suffocation and should not be used.