This arc isn't THAT bad imo. Just boring and plain. It's the last big action arc in the story, but it doesn't bring anything new or refreshing to the narrative. Generally speaking, every arc has followed roughly the same structure so far:
1. Rokudou meets a bad girl, she falls in love with him
2. The bad girl being a bad girl, has other bad people around her; these bad people she knows usually tend to become the antagonists that act as obstacles for Rokudou and friends to overcome before our protagonist can reform the bad girl
3. The arc usually has a big group fight as its main climax, with Rokudou ultimately persevering and coming out on top; after this, the bad girl's beliefs change, and she joins the gang as another friend
But, every arc so far has had its own unique charms, despite following this general same formula.
The first arc — where Rokudou gets his powers and meets Ranna — was obviously good because it was our first time seeing his magic in action. We got to see what kind of person Rokudou, Colonel, and Kachou are. We also get to see what the girl known as Ranna, is like. But, most importantly, this arc is dedicated to Rokudou growing a spine and deciding to change his cowardly ways. Him standing up to Iinuma and getting Iinuma's respect was the main climax. I still have that panel of Rokudou crying like a baby in the thunderstorm outside, as one of my top moments in the entire series.
The second arc introduces us to Osanada. I won't go into too much detail for every arc, but I liked how genuinely heartfelt this arc was. The main theme here was Rokudou not wanting to ruin Osanada's first love. Between Rokudou's class doing a flower observation project together, and Osanada softening up after meeting him — this arc was super emotional, and pulled at my heartstrings.
The third arc, Azami's arc, was very thrilling too. The action sequences revolving around high-speed motorbike chases was peak stuff. Azami and Aoi's backstory with that cop was quite touching as well. Another awesome arc.
The Onishima Alliance is the first "Dungeon Rush" arc we get. Because it was the first time, it was cool. Everybody dealing with each of the Alliance's Top 5 lieutenants, Douji trying his best to twist Ranna back into her old evil ways, Rokudou stopping her, our main pair having a talk on the rooftop in the middle of the final fight...Also a great arc.
Minami's arc was a fresh change of pace. Because the main theme of this one, which was money, gave things a sadder and more mature feel. Rokudou gets his first glimpse into how bad the world of "adult delinquents" can be. If high schoolers just bully each other and get into fights, then adults can do much much worse — especially if there's money involved. This arc touches the most "realistic" topics, given the bizarre nature of the ones before it.
For me personally, Ling Lan's arc was the best in the series. Rokudou finally winning a fight was fantastic. But, more importantly, you can see the author's love for Chinese martial arts media being shown in this arc. The outfits, the references to Hong Kong martial arts movies, Bruce Lee, the whole atmosphere of learning kung-fu...Super awesome stuff.
Yashiya's arc with the Chromosome felt more like a filler arc. Had action, sure. But the constant genderbending/crossdressing made it feel more like a lighthearted comedy arc. Not to say that it didn't have its moments, though. We get a look at Rokudou's character motivations grow, with him wanting to be more "manly" and for Ranna to love him for who he is — without his magic (Ling Lan's arc also shows this too, which is nice). We also get a closure to Raino's feelings for him.
The thing with this arc is — it just doesn't do anything new. Rereading it without having to wait for new chapters — I wouldn't say it's badly paced or dragged out. It just feels like this arc is slow because it's honestly a bit boring. Frankly, it's like a plain "save the princess" trope turned into an entire arc. The other arcs have Rokudou saving the "princesses", but they all have their unique moments. This arc just feels like a vague rehashing of the Onishima Alliance, but slightly different.
The stakes are raised here at first glance — the gang are now actually dealing with real-life Yakuza. Grown criminals. But, that's only on the surface. The main villains here don't get any real characterization, and just feel like more bad guys for the teen heroes to beat up and prevail against. Shallow, basically. At best, they each have one or two of their own little gimmicks — and that's it.
The "gathering all the old friends and foes" thing feels kinda contrived/forced, too.
Overall, the ending messed up a little — but it's clear that the author was just pushing himself to get one last action arc out of himself, before the big mystery with Ranna is resolved and the series is finished.
Again, wouldn't say this arc is bad, though. Just boring, repetitive. Unnecessary. If it WAS a bad arc, then we'd be seeing character regressions. Which haven't really happened so far.