I'd say, if two things have something in common, you can compare them in terms of the common ground. As for non-linearity of the games, they were all linear to a greater or lesser extent. They all had a plot that only developed after key events/actions had been negotiated, and the plot always developed into one of a limited number of permutations. They also all had a fair number of things you could do which were nothing to do with the main plot, or not integral to it. It could be argued, especially of Fallout, that the games allowed the character to take different paths along the main plot. I'd say this is multi-linear. For a non-linear game, I think there would actually have to be no main plot, or one so vague (i.e. such a huge number of different paths that developed in subtle ways) as to be unnoticable for most of the game.