In response to a comment on my previous post, I posted a question on the original tbw thread (where I got the bag sling info) on how to differentiate between the "bad" Infantino-style bag slings vs. the mei tais, pouches, and ring slings I love to recommend...this was the response:
Answer #1:My answer would be: YES, correct positioning is always important, however, in a sling without all that padding and extra thick fabric correct positioning is much easier to attain since you can see exactly where your baby's body is positioned, and if the airway is clear, since you have an unobstructed view of your infant, unlike in the "bag slings".And answer #2 (from the blog author, M'Liss, responding to the above quotation):
Exactly

THE number one concern I have with bag slings is that it is so, so, so difficult to see a baby inside one - unless the parent opens it up every minute. For instance, if baby spits up and starts to choke the parent wouldn't know. For comparison even back carries in a wrap are, IME, much safer because a newborn spitting up on your back - well, you'd know pretty darn quick

And in other TBW type carriers you can feel the baby move, breathe, etc. Bag slings - no clue what is happening to baby inside the bottomless pit.
Now to get back to the blog question. The person that posted does have a point about any carrier, used incorrectly, having the potential to be unsafe. But therein lies the difference.
Used incorrectly or
designed incorrectly. Following the bag sling instructions to the letter will not make the sling any more safe or comfortable because the
design is seriously flawed.
Does that help??