Are they dangerous? You bet.
In addition to falls down stairs and falls out of their walker, many infants are injured each year as their mobile baby walker makes them a little too mobile and able to get to things that would otherwise be out of reach. This may allow them to reach countertops and get burned or poisoned by things they pull down, drown by falling into a pool, bathtub, or toilet, or simply hurting their fingers and toes if they get pinched.
Background
The number of injuries from baby walkers recently led the Canadian government to ban the 'sale, advertisement and importation of baby walkers in Canada.' Although they haven't been successful, the American Academy of Pediatrics is urging the US government to do the same. Instead, the Consumer Product Safety Commision has been promoting new safety standards for baby walkers that can lead to fewer injuries, especially from falls.

