BioWare has a very good understanding of how to give the player just enough freedom to believe their actions affect the stories outcome, without actually affecting it.
I’ll admit that in the Star Wars: Knights of the Old Republic games, you had the ability to align your character with the light or dark side of the Force. But the story canon only considered the light side the victor.
In Mass Effect, not so much. Doesn’t mean they aren’t fun… Well, at least until the very end.