I'm not really familiar with the Yakuza series, having played only Yakuza 0, 5 and 6. Of these, I must say, the overarching prequel - Yakuza 0, is the best.
Yakuza 0 is a lot of things, but first and foremost, it's an expansive, ambitious melodrama. It's surprising how deftly Yakuza 0 combines humour, pathos, and mystery into one massive story, told across two protagonists. The plotting is labyrinthian, with twists and turns a plenty. Just when it seems like they've gone too far - it all comes together brilliantly, and kept me engaged till the very end. I did mention it's a melodrama - some will automatically take that as a criticism. But good melodramas exist, and in the tradition of Japanese cinema, Yakuza 0 is a rare example of it. The characters are delightfully written, particularly the protagonists. They do broadly fit the expected archetypes though, so it's very much conservative in that regard.
Yakuza 0 is a cross-gen game, and it shows. While it can look pretty at times, it definitely feels like a game stuck somewhere between a PS3 and PS4. Further complicated a rather inconsistent presentation style, juggling between 4 or 5 different types of cutscenes seemingly at random. There are tons of cutscenes, so the transitions between the types of cutscenes is always immersion-breaking. It definitely doesn't feel as polished as its successors. The character models are fine, but the animation is clearly not up to modern mo-cap standards. Still, thanks to the strength of the dialogue and storytelling, it's never a major distraction.
To be overly reductive, the game in a nutshell can be summed up as - watch a cutscene, find yourself in trouble, beat the shit out of everyone to find yourself out of it. The game is very well humoured about it, though. The two characters have vastly different fighting styles, with differing styles within each character. Some of these styles are ridiculous - especially Goro's dancing style.
Not an open-world per se, Yakuza 0 features multiple hub areas. There are a fair amount of side quests. Though most of them are of the generic fetch quest variety, there are also some hilarious ones. It's hard to tell which ones are going to be fun, though. Then there are all sorts of minigames, littered across the world. Needless to say, each of these are pretty basic and not really fleshed out, but it's surprising that there are so many present at all. There are two major side-games apart from the minigames - real estate management and club management for each character. These are definitely more detailed than the regular minigames, with their own mini-quests. It can be a bit of a grind, but worth completing.
Try as it might, though, Yakuza 0 always returns back to its basics - tons of cutscenes followed by some good old fashioned brawling. Overall, given how expansive the narrative is, I don't think the core gameplay mechanic offers enough progression or variety. After about two-thirds through the story, it definitely starts feeling repetitive. Still, the strength of the storytelling kept me going. By the way, I'm not sure how no one ends up dead, given how brutal some of the special moves are...
For a game that's mostly just a bunch of cutscenes followed by a bunch of brawling sequences, Yakuza 0 ends up quite the kitchen sink of a game. The expansive story throws pretty much every emotion and narrative trick possible, while the brawling experiments with outlandish ideas. The world and minigames add a further layer on top, with a huge variety of activities. It all adds up to a delightful package that somehow feels more than the sum of its parts.