I'm going for a complete NA 360 set myself (about 400 games left) and El Chavo Kart is definitely the holy grail of what's left.
Out of those other games you mentioned, I have Deadfall Adventures, but it wasn't difficult to find at all (5 bucks from Amazon's warehouse deals). Really, the biggest limiter is that I can find the titles, but I either don't want to spend that much or would rather use my budgeted collecting money towards something else. For me, a great example for the 360 is College Hoops 2K8; I can find it easily, but I can't justify spending $25+ on it, when I've got plenty of other titles on the wishlist to choose from and also when the other games in the series were obtained for less than 5 bucks apiece.