Not only is there a risk of suffocation, but SIDS has been linked to lack of oxygen circulation (which is why they suspect children of smokers are at a greater risk for SIDS). The bumper main reduce circulation around the baby.
When they are very little, they don't maneuver enough to get their limbs tangled. As they get older, it might happen, but that's not a life-threatening thing, it's just a stuck arm or leg usually, and they'll holler and let you know very quickly that it happened. At that point, if it is a recurring issue, we will consider a breathable mesh bumper. Otherwise, just not interested. We didn't use one with our son, and I was glad it was one less thing to wash.