The point of the Cares harness is to make the lap belt safer for children who may be small enough to slip out of it in the event of a rapid decent or turbulance (a baby could slip out of your arms). I have traveled with the Cares and with a car seat (our big ole Britax). We used the car seat at 11 months and 23 months. One because we needed it when we got there and I was not checking something that could be ruined if dropped, and second our daughter did well in it because it signaled car ride. A car seat is a pain, no way around it but it was still a minor inconvenience.
We used the Cares for our first ME trip at just shy of 3 years and it has worked great. No problems with the tray tables behind us. One woman didn't even know I had installed something, she thought it was part of the seat. They do slide some when younger because there is no belt between the legs. I get better and better at installation. Try it out at home. I bought a used one off ebay instead of renting, and I plan on selling mine that way once we are done with it.