Unlike bumpers, crib rail guards are secured at the top of the crib, above your child's sleep environment. Because of this, crib rail guards are much safer than bumpers.
They also serve a different purpose. Rather than protecting against bumps and bruises, rail guards protect your teething baby's gums from the paint, wood or other materials the crib is made of. (It also protects the crib from you baby's gnawing nature).
While they're generally deemed safe, it's important to carefully choose your rail guards to make sure the ones you choose don't pose any problems. If you opt for guards that tie into place, make sure the ties never hang low enough for baby to grab. A strong tug could cause the guards to come loose and fall into the crib and become just as dangerous as any other pillow-like objects. Seek out ties that are nine inches or shorter.
Steer clear of rail guards with cute buttons; should these come loose and fall into the crib, they'll pose a choking hazard. Snaps are safe, but if your crib has curved or thick rails, they can be tough to keep secured.
Since your baby will likely try to ease her aching gums by chewing on the cover, make sure it's made with a durable, washable material, and ideally one with a waterproof backing to protect it from all the drool (you'll be surprised as to how much drool your baby will produce).