Taken in isolation, Assassin's Creed Pirates is a fine but not fantastic game--the kind of mobile experience that's enjoyable enough for what it is, even if it is somewhat forgettable. Placed in direct contrast to Assassin's Creed 4, however, its flaws feel more glaring.
The pirating mechanics have been significantly altered from Black Flag, and always for the worse. Rather than free-steering your vessel to flank or ram ships, getting close to one puts you into a side-by-side combat view, as both ships stay perfectly parallel with one another. You can still aim your shots, but usually will only get off one or two before switching to a dodging mini-game for the opposing shots. This half of combat is somewhat comical, as your pirate ship will pull forward or back so quickly it doesn't even seem like a decent attempt at realism.
And so combat goes: back and forth, taking a shot or two and then dodging, with no real challenge in either case. Gone is the ship-boarding or taking fortresses. You are still able to freely steer between objectives, or follow behind a lead ship. An overhead mission type will have you trace lines to avoid being spotted by enemy ships. Those give Pirates a little more variety, but they're poor substitutes for the elements that been removed.
The story also feels just as uninspiring, following Alonzo Batilia, who is taken under the (aggressive, mocking) wing of the pirate La Buse. The pair lack any real chemistry, and it doesn't present great reason for Batilia to even be given his own ship. He seems more like a thorn in La Buse's side than a friend. The sparse plot is delivered with quick comic-style cutout characters, who don't look particularly well-drawn or fitting for the style.
It is a very attractive game in other respects, though. The ships are well-detailed for a mobile game, and the water and environmental effects are almost on-par with the console versions. It could serve as a subtly striking showpiece for mobile platforms. If only the gameplay were more exciting.
Pirates isn't a terrible game, but its pedigree means it needed to aim higher. Black Flag's ship combat was extremely refined, but Pirates comes off as a shallow imitator, and will undoubtedly leave players wanting. 
Steve Watts posted a new article, Assassin's Creed Pirates (iOS) review: shallow waters.
Assassin's Creed Pirates is Ubisoft's companion piece to Assassin's Creed 4, but does it do justice to the refined pirating mechanics of its older brother? Our review.