Double checked on this...and they can be used on a child weighing more than 22lbs. Completely safe. I' m booking one off of Ebay for our trip.
We own a Cares harness. I usually use it for my daughter, who is 4.5yo, but in June I used it for my son, who was 22 months, 25lbs, 34in. He was very uncomfortable in it. He fell asleep briefly but his head was hanging forward, he ended up sliding down in the seat so his neck was practically touching the chest clip. He woke up and was grouchy and annoyed. The only reason I used it was that we were in the airport for over 6 hours with a delayed flight, I was alone with both of my kids, carrying a ton of crap with me all day long, and I just didn't feel like carrying it off with him, and possibly my daughter, asleep when we landed. Big mistake as he ended up not sleeping, grouching out loud most of the flight, and fighting to get out of the seat.
Based on that experience, we brought his carseat last week and he slept in it on both flights, the whole flight.
So I would think about the age of the child, not just their weight, when using this.
That said, I think it's a wonderful invention and use it with my daughter (and have since she was 3yo) without issue. I just think my son needs to be older and able to sit properly in the seat and understand he needs to stay there before he can successfully use this again.
Oh yeah, and we fly with Cosco seats. So much lighter than our Britax and very easy to install on the plane.
And as for rearfacing, both of my kids were/are extended rearfacers (my son is 25 months and still rearfacing), but on the plane we turned them forward facing probably around 18 months-2yo? In our case, we did it because some planes the seat pitch was simply too shallow to allow a comfortable angle for our kids (Delta comes to mind here) and in other cases because I could not get a good rearfacing fit. Southwest I had trouble in the bulkhead as the seatbelt positioning did not allow the belt to stay tight rearfacing (it was right on the corner of the seat, so it wasn't laying flush). If I could have gotten the seats in snugly and comfortably we would have kept them rearfacing on the plane, but it just didn't work so I figured forward facing was better than no seat.
Oh yeah, last thing I promise! When my daughter was around 19 months, she got around the 'kicking the seat in front of her' while rearfacing in her carseat by pushing against the seatback of her seat, thereby slamming her rearfacing Britax Roundabout into the seat in front of her.

Let me tell you how much I enjoyed that flight, having to restrain her legs and the seat!