Like all good, not great games, Assassin's Creed 4 offers a mixed bag of good and bad. The game is well executed. The music, the graphics, the sound effects, the writing, and the dialogue are excellent.
As for the bad, these are problems that have compounded over the course of the series. The series began with a flawed but promising idea of mixing parkour and stealth, along with neat concepts melding modern technology with historical appreciation. However, in that first game it seems like they had trouble bringing elegance to the gameplay. The biggest single problem has always been control, flimsily dependent on the camera, and so the solution to this was just to lower the difficulty to limit the frustration of it. I loved that game because I appreciated what they were trying to do, even if they didn't succeed in everything.
Now, all these years later, they have made zero progress developing the core mechanics. Black Flag is actually the worst in the series in terms of fundamentals. The control is extremely imprecise now. The parkour is unchanged, as bad as ever. The new diving bell gameplay sequences are a nightmare in terms of camera and making your character swim in the proper direction. Not even the ship sequences have control precision, and they are a completely separate mechanic! Specifically, you can fire ammunition in 4 separate directions, and depending on where the camera is it is extremely easy to use the wrong attack. The parkour elements of walking up and down the masts is a disaster as well. Buildings and rooftops are manageable. The series of posts and ropes on the masts are virtually impossible to master.
Ubisoft's answer to all the frustration of the controls is always the same: just lower the difficulty. The game literally feels like a 'very easy' mode of a game with variable difficulty. This is common for the series, the expanded gameplay elements use this strategy for every problem and it's become ridiculous at this point. There is no challenge whatsoever for the ship-to-ship combat (except for the Legendary Ship side missions). The diving bell sequences are kept relatively short and forgiving. Admittedly, the harpooning mini-game is okay, but it is by far the shortest and smallest of the new gameplay additions.
Having said this, overall the game was enjoyable. There are a lot of meaningful collectibles to be had, ones that let you upgrade your character and your ship in meaningful and fun ways. Level design is generally strong and the stealth elements of the game are satisfying, even if you have to limit yourself in order to be challenged by them. Anyone will be able to appreciate the care and craft that went into building the world and in the presentation.