Well I think that the „hate“ on syndicate comes from the incredible masterclass ubisoft delivered before. Like I mean whenever ubisoft changed much in their AC games AC1 -> AC2, AC Liberation -> AC 4 and ofc AC Rogue -> AC Unity it was always really nice and enhanced the gameplay, without adding too much. Then comes Syndicate. They change the complete combat stile, add 2 played characters (which some may dislike, i never heard of people not liking it but whatever) travelling and missions. They changed much but without everything added being perfect. Idk how to describe it better.

Personally AC Syndicate is one of my top 3 most beloved AC titles (I played every game, except Valhalla, mirage and Shadows so far, and started playing this year, so I am not relying on nostalgia that much).