I've been using them for years and use one now for C. The crotch strap is a bit short but it's designed to be that way and I believe Graco said at one point that it's ok if your child is sitting on the slot where it comes out a bit, as long as it's in the furthest slot away from the bight of the seat. I usually just have kiddo lift his bum up a bit so I can pull it out. As far as head slump, it's going to depend on your car. MOST combination seats don't recline a lot so as combination seats go I actually think it's pretty good. But I have seen some head slump depending on how it sat in my vehicle seat (I've used it in at least four different vehicles). The most important thing to remember with that though is that for older kids, head slump isn't dangerous, and if they're sleeping, they're probably not even uncomfortable - it just looks that way. The Nautilus DOES have a recline feature which is NOT particularly common in combination seats, I just don't think it makes much of a difference honestly whether you use it or not.
I really like it. It's my favorite seat, actually. I'm trying hard right now to figure out how to get three across in my tiny car with a Nautilus in the mix!