An underrated title in the series. Gameplay is mostly Black Flag, but more refined. Story is meh, just reversing the roles of Assassins and Templars (my headcanon is, Abstergo is altering the memories being played, to make the Assassins the bad guys of the story). Overall, a solid 7.5/10 and a must play in the series.

remove this from steam, it along with any "old" AC games do not work at all, it has been this way for months.

I came to this (as I guess most people will) from Black Flag. This feels like an unfinished DLC from that game, and although it shares many features with BF, almost everything about it is just... less.
Combat, both at sea and hand-to-hand, is fine. Pretty much the same, but feels rather less challenging.
There were bugs I haven't encountered in previous games, and a handful of times I had to lose my progress and reload because I got stuck at some random point on the map.
The story, though. Ugh.
Ezio had a powerful motivation. Edward Kenway started out as a mercenary figure, but built real connections to his pirate friends and you came to care for him to the point where the ending was genuinely emotional.
Shay Connor doesn't do this. I didn't really believe in his motivation to make the choices he made. I didn't feel he built up any real friendships, just a series of functional collaborations. I just didn't like him.
The game didn't flow, either. You jump back and forth from one location to another without any real goal or direction. So you follow the main storyline for a while to try to make sense of it, and suddenly you're trapped into playing the ending even though you're only 60% of the way through the game, and other things you've been working on are left dangling.
Even the modern-day sections don't carry any of the intrigue or conflict of those in BF.
It all just feels a pale imitation of its illustrious predecessors. If you pick it up on sale, you'll get some hours of distraction from it pretty cheap. But I never felt compelled to get back in there and find out what happened next. There are much better ways to spend that time.