Its not so fundamentally different from any other game in the AC franchise to garner so much hate.
I think its just a trendy thing to say, with no real thought given to it.
I think according to some people on the internet, anything a lot of people dislike "is just a trendy thing to say with no thought given to it". I hope you don't dislike anything unpopular.
1. What I saw of the story was dull even by video game standards. I suppose story quality is subjective, but even the voice actors sounded like they were going through the motions. I can't speak to the quality of the story on the whole, since I didn't finish it, but the mark of a good story is the need to "see what happens next", and I didn't feel something interesting was around the corner.
2. Den defense was out of place and a feeble attempt at innovation (although it least it was an attempt). I play AC games to be a stealthy badass, not a rooftop general.
3. Being punished for expanding the economy may have been "realistic", but it wasn't fun. It made it even more of a time sink.
There were other factors (such as screen tearing) that bugged me, but those three forced me to quit not out of frustration, but boredom. I had other games I wanted to play, and other things I wanted to do. It happens.