I don't really think it's dangerous, as long as the door is sufficient in height. There is the off chance that they can get objects into the stall, but if nothing is left for them to get, that should not be an issue. Amber did used to pull her bell boots off her blanket rack and kick them around for fun when she got bored, and that was annoying, but not dangerous to her.
If the openings are close by the other stall, there may be some nipping conflicts with other horses though...but again, that's not terribly dangerous. I'll put it this way - there are circumstances that can make it annoying, but it wouldn't turn me off of an otherwise good barn, and it wouldn't factor in as a turn off if I looked at a bad barn.
Cinnamon Whiskey 11 y/o 15hh Chestnut AQHA mare, 2'6 Jumpers