The concern is over the bag slings ONLY, yes. Though of course whichever sling you use you need to make sure that baby is positioned correctly and well supported.
There is lots of excellent information on this facebook group
http://www.facebook.com/?ref=logo#!/BabywearingSafety?ref=ts
There are entire cultures where prams and pushchairs don't exist and
babies spent their lives strapped to caregivers until they can
literally stand on their own two feet.