I feel like I'm repeating myself here, but here goes.

Usually people who say every game is exactly the same are people who have not played all the games in their entirety. There is no way that AC Unity is remotely similar to AC Rogue or pretty much any AC before that (except maybe the first game, there are some similarities there). There might not be major differences between some of the titles at first glance, but I can assure there is enough different content in each title to make is stand apart from the rest. The settings are also some of the best things about an AC game. I really enjoy the different historical settings and how they work historical figures into their fiction.
I understand that people feel the games come out too soon after one another, but let's not forget that Ubisoft have multiple studios around the world working on AC and each game has an average development cycle of about 3 or 4 years, if I'm not mistaken. I'll admit, sometimes the multiple studios working on a single title doesn't work so well.