There aren't many sci fi films back in that time because it was so "impossible" to pull those sorts of effects off. In Terminator, which Cameron also directed, they did use some CGI that existed at the time like in gun-fights with laser guns or whatever and those are the only parts of that film that look cheesy. I'm really happy that they used practical effects in Alien and Aliens, this is why they are good. Alien 3 used CGI, and the drop in overall quality is apparent right from the start.