I think there are a couple of reasons that the Complete Air isn't more popular. First, it has a 40" height limit for RF, even though the vast majority of children could fit RF well beyond 40". Some parents are comfortable ignoring the 40", but it makes it hard to recommend when there's such a limit. Another reason is that the CA is supposed to be installed with the line on the seat level to the ground, rather than in a more upright position. In some vehicles, trying to install the seat with the line level to the ground makes for a super-reclined install. I think some of the new CA seats have dual lines that allow for more upright installs for older kids, but it's still a limited install range. Some parents are comfortable installing it more upright regardless of the lines, but again, it becomes problematic to recommend.
Additionally, I believe that it has continuous loop harness, and some people prefer seats that have two separate straps (I know the continuous loop harness on my True Fit drove me bonkers).