I think the problem is that Ridley Scott's 'Alien' was not only the the greatest Sci-Fi/Horror movie ever committed to celluloid, but also one of the greatest movies, of any genre, of all time. Therefore a movie that built upon its legacy was always going to have comparisons drawn. It was sure as hell going to be judged harshly, and it is perfectly understandable as to why.
However, in saying that, the movie is an enjoyable B-movie romp that stands on its own two feet. It's well worth a watch. I just hope you understand a little more now as to why this spin-off franchise is judged so harshly. One can't help but feel it is a little bit of a travesty to see our glorious 'Alien' romping around in a fairly low budget b-movie. But that is not the movie maker's fault....it is whoever signed the licence to allow it. Had they made the same two AVP style movies, but with two unique alien monsters, I think it might have even faired better in reviews. But there again, the names of 'Alien' and 'Predator' have probably shifted a good few million DVD's for them.....so as usual, money is the answer to that little mystery.