At its core, Nanbaka is a zany, colourful gag comedy in a prison setting! ...Or is it an action shonen? No? A character drama? In practice, it frequently switches targets when it may have been wiser to stick to one.
From the get-go, Nanbaka presents a fast-paced series of varied jokes, ranging from visual humour to ironic subversion to borderline non-sequitur. They weren't exactly original, and didn't get to me personally, but variety and quick pacing are handy tools for holding interest whenever a punchline doesn't quite hit the mark. At this point, I respected Nanbaka for what it was. It had a clear identity, and showed commitment to it.
However, the show quickly lost its focus from there. In my eyes, a lack of focus is one of the fastest ways to kill viewer interest, and often a sign of poor writing. Nanbaka soon falls into this trap; rather than continuing its gag comedy roots, it tries to take on multiple identities at once. As soon as the action and drama arrive, the pacing slows considerably and the jokes become less varied or frequent. This would be fine if the new elements meshed well, or were at least strong additions to the series, but instead, they just make things messier.
The action elements suffer from weak choreography. This is expounded on by mid-action cuts to narration by onlooking characters - an issue seen in many action anime. For once, Nanbaka could have made this trope work in its favour by filling the narration with its usual zany jokes. Alas, it's just the usual filler dialogue to stretch out the no-longer-action while explaining attacks that didn't really need to be explained and/or stating things that are obvious to the viewer.
Despite the action portion's weaknesses, the drama is where I find Nanbaka to be the least successful. At these points, the show suddenly takes on a more serious tone than the accompanying gag comedy and action. This results in a weird situation in which a viewer is simultaneously expected to take the show seriously while also not taking it seriously at all. It's not the tonal shifts that are the issue - rather, it's their frequency and the show's inability to sell them.
Furthermore, compelling drama requires compelling characters - something Nanbaka does not have. Most of them come down to one-dimensional quirks or hobbies and maybe the occasional simple backstory. They're too simple to be interesting on their own, and it makes investment difficult.
And this brings up what I believe to be Nanbaka's main weakness, and the source of nearly all its problems: it's characters. Even with its wildly differing subject matter, this show certainly wasn't doomed from the start. It simply lacked the tools needed to succeed. While there were several things that could have helped the show (improved action pacing and choreography, or a more memorable soundtrack to quickly and effectively shift tone), ultimately, I believe that characterization is the one key missing weapon it really needed to hit all its desired targets.
Stronger characters can create chemistry which helps to sell the comedy and elevate it from "gag" to "riot". In turn, these jokes can be a quick and effective way to show characterization. From there, these improved characters help to invest a viewer before the show switches to action, and when drama builds, the viewer will more readily buy into it. But without this depth of character, the act falls apart, undercut from its very foundations.
Despite all my criticisms of Nanbaka, one positive I can mention is the show's art. It has a unique, sparkly, insanely colourful style that immediately draws attention. I'll admit, this alone made it easier for me to sit through Nanbaka's weakest points. The style, however, is not nearly enough to carry the show on its own against its many weaknesses.
In the end, Nanbaka gets a tad too ambitious for its own good, and misses the fundamentals as a result. What could have been a notable series sadly ends up as a disorganized, zig-zagging mess, with no clear identity beyond its visual style. When it does manage to catch a rabbit, it's not a particularly tasty one.
