So, I've played a fair bit of the game over the past 3-4 days and have just completed the main story as well as a fair bit of the Treasure Log mode (up to New World rank now) so I've got a good grasp of its quality overall. The story isn't much of a drawing point with it only containing a few major arcs and having a non-canon ending, but that's to be expected when the other choice was having it be a long slog to get through like Pirate Warriors 3. That said, I would have preferred it if the game just went the route of Pirate Warriors 2 by having a completely original non-canon story if it was going to half-ass it anyway.
For the actual gameplay, it's probably the best out of the PW series when it comes to combat. The level up system has much to be desired compared to 3 though as it goes for a more limited skill tree route rather than typical levelling up that requires certain coins to max out some characters. The roster is also fairly limited as it took out some characters for the sake of adding more recent ones that show it catering more towards recent popularity rather than story impact. For instance, Enel, Moria, Kuma, Garp, etc. don't make the cut (despite the latter 2 still being NPCs) yet Carrot and the Vinsmoke Family apparently have a great enough move pool to make it in as playable? Another give and take is to the usefulness of certain characters. Ace and Marco got a good buff, yet Whitebeard and Fujitora got nerfed quite a bit. Another problem is with certain displays. Unlike the previous game, you can't check the rewards for increasing the crew levels of characters from the gallery and instead have to memorise what bonuses you get for certain characters, which is annoying if trying to break a character's level limit. For instance, I'm trying to max out Luffy, yet don't know who's bonus gives you the Devil Fruit coin.
Still, it is quite a fun game to play, though a very flawed one.