A lot of people rate Black Flag as one of the best AC games. I found it on a level with AC 3, but with much better naval action.
Characters and character acting. Edward is one of the best main characters I have played in AC. There are also other standout characters amongst the privateer captains. Characterisation overall is better than any other AC game.
The story is okay, but Ubisoft are just not that good at story telling. They don't make enough use of characters as individual people, there is no sense of time passing, and they don't help the player to understand the plot. Consequently much of the time you are assassinating people, sinking ships, or completing missions without a clear idea of how it all pieces together. For me this is a problem they did not solve until Odyssey, where the player-controlled destruction of the cult draws everything together.
The naval action is fun, but no where near as accomplished and addictive as in Odyssey. I disliked the additional boarding objectives. They become annoying because they show up how fiddly the control system is. I don't understand why they fitted rams to ship's (not historically accurate), and I was disappointed they did not make ship battles more strategic. The ship's sail like motorised vessels with not even a nod towards wind direction. The simple introduction of wind could have transformed the naval battles.
There is a problem with the PS4 control scheme in that the R2 sprint control on a DS4 does not work. This is exactly the same bug as in AC 3 and Liberation. It should have been sorted. For any chase sequence I had to keep swapping to my spare controller, and then back again afterwards because I prefer the DS4s ergonomics.
The stealth is quite good - hiding in undergrowth, merging with crowds etc. But why is there no crouch control? It is immersion breaking because you cannot actually sneak - you walk upright between hiding places. I never got used to that. I disliked the reliance on 'tailing' and 'eavesdropping' tasks in missions. They always took several attempts because they have to be done in a specific way with little allowance for mistakes. It just doesn't fit with the open world nature of the game - it's no longer a tactical task, but just one of learning where the game wants you to go.
After I completed the story I was not motivated to play with the end game content, or even the online content, because It is all so grind-heavy. Shipwrecks are an annoying lottery where lack of air and multiple sharks and eels mean constant replays. Contracts are unengaging "go there sink this" tasks. Even upgrading your ship is a pointless ship-sinking grind, and is mandatory if you want to attack the legendary 'boss' ships.
To be fair, the game provides plenty of end game content, and that's even before you consider the on line element. I can see how it is attractive to MMO style players who are not afraid of a grind to achieve an optimum build. But if you are a mainly solo game player, it will not appeal - even going for a platinum will demand a huge commitment in grind time.
Overall Black Flag was a disappointment. I'd say it was better than AC 3 because the naval element provides more game play variety. But I preferred Origins (you can at least go for the platinum), and it is way below Odyssey in every department.