I think it's partly because the animations and voice acting were so glaringly, comically bad at launch that it mired the release, just as it did for Assassin's Creed Unity, even if it didn't affect everyone or was as bad as the media were suggesting. Also Mass Effect is one of the most treasured series' of all time. If it's not done perfectly then people will be p***ed. It had to be amazing or it was going to be a failure. Fallout 4 almost suffered that fate. It was a divisive game, but mostly because of Fallout's heritage, not because it was a bad game. If Fallout 4 had been a completely new IP I think the Steam reviews (just as an example) would be Very Positive instead of Mostly Positive.