Everything! I need to say right now: NEVER back carry with a stretchy wrap.
Yes, it is technically possible.
No, it is not optimal.
No, it's not really safe.
Now, here's the confusing part:
Wrapsody stretch hybrids CAN be used for front, side/hip AND back carries. Why? Because they're a stretch cotton with WIDTH-ways stretch but very little LENGTH-ways stretch. They won't 'give' the same way a true stretchy will, meaning your child can't backflip out (trusting you have a decent wrap job, of course!)
So why not just buy a woven, or just buy a regular stretchy?
The hybrid will give you more support (and therefore can be used for longer as your child grows) and it's a little easier to know when you have it "tight enough". I don't know about you, but I find it difficult to gauge when a stretchy wrap is sufficiently tight - part of the reason I've always favoured woven wraps and ring slings. However, this is much softer than most (new) wovens and would be lovely with a new baby. It's also going to be more forgiving for a new mum who is learning to wrap, as it's gentle on the shoulders without really careful tightening. Put it on the top of your list for new mums - a baby carrier they can use comfortably and safely for at least 12 months and beyond.
Let me state that Audrey is approximately 10kg, or 22lbs, and I easily back wrapped her and had her up in back wrap cross carry for about an hour before I felt the need to adjust my wrap job. We later FWCC'd to sleep, and I can see this being a perfect lender wrap to enable new mamas to get comfortable with wrapping at a great price point.