Those who didn't like the movie...

It's just half of it. The games always prioritized historical events first, present day backstory second. Games mostly had the Forest Gump effect where the protagonist is a fictitious character we see the events from the eyes of. The historical elements have always been based on reality. The film threw in the historical elements there as a gimmick, to be used as a fantasy element.

It was quite baffling to see that the game franchise itself was far more grounded in execution and more believable. Even for true AC fans, I think the movie took itself too seriously and played it too straight. It was embarrassing and even disturbing at times to watch those professional actors delivering those corny lines with no expressions on their faces. The film pretty much introduced the assassins as mindless psycho occult freaks, which is probably closer to what they were in reality, but objectively looking at it, I haven't seen any good reason why I should find them sincere and feel sympathy for them.

I feel like they tried too hard, made things unnecessarily complicated. For example, the first Animus in the game was a bed, then a chair and finally head sets attached to desktop computers. And more or less, you're convinced of how they work. I really couldn't wrap my head around the device in this film. I mean, I can see why they did it! They needed more action than Fassbender chilling on a bed or a computer, but again, it unnecessarily complicates things. We needed a simple story with simple devices. Instead, the film bombards you with crazy action, unwarranted dramatic shots, characters rushing to explain to the audience the lore and the plot, and that ear shattering background music that just won't take a pause. All of it simply made me ask "Why?" and I couldn't find a convincing answer!