I've had a Baby Bjorn and a Baby Bjorn Active and I seem to remember they weren't supposed to be used before a baby is about a week old. But we weren't going anywhere at that point.
I certainly don't think they're dangerous and I think they're one of the better mass-produced makes but they do have limitations.
They don't last very long because they become uncomfortable for the wearer at around four to six months, as all the weight goes on to your shoulders, whereas other types will spread the weight to your hips and waist.
They also only allow you to carry the baby upright, so they aren't as versatile as a wrap sling, pouch or ring sling which allow various carrying positions.
The people who sell them don't generally have a clue about how they should be worn - high and snug. I always wince when I see parents with them dangling around their stomachs because it is going to hurt their backs.
And if you wear them with the baby facing outwards it may look cute, but it isn't supporting the baby's back, it's putting an extra strain on your back and the baby can't get away if the outside world gets too much for the moment.
They are also pretty expensive for the use you get out of them.
I wouldn't say don't use one but I would say make sure you wear it correctly and be aware it won't be comfortable for you for very long.
A fabric sling would be more versatile, would last longer and will hold its value better if you want to sell it when you've finished with it.