Before we talk about Senran Kagura: Ninja Flash!, let’s talk about “Dumb Fun”.
There’s an idea prevalent in the western anime fandom that Japanese people do not know that certain media looks silly on a surface level. In truth, it’s more the fact that westerners (and Americans in particular, speaking as one) tend to have an underdeveloped sense of Camp. We tend to default to assuming “dumb” Japanese shows are in some sense naive; that they somehow don’t realize that they’re making something goofy. Hence the “dumb fun” label and the “turn your brain off” refrain. In anime, this generally not actually the case. Shows like say; Kill la Kill, PUNCH LINE, or the Symphogear series are campy to be sure, but they have genuine emotional cores backed by solid character writing, and deeper themes if one cares to pay attention to them. Fun, yes, but not dumb by any means.
Of course, nothing in life is ever completely clear-cut.

I say this because “Dumb Fun” is a term that fits Senran Kagura: Ninja Flash! like a glove. Even if it was your very first anime, it would strike as cliche. “Ninjas” who are in reality super-strong schoolgirls in skimpy outfits, cleanly divided into a “Light Faction” and “Shadow Faction”. A narrator who, straight-faced, solemnly explains that a shinobi’s armor is made of her ideals, and that being struck physically or harmed mentally can cause it to disintegrate, leaving her in her skivvies. A plot that can be boiled down to “our heroes get beaten by the villains, train, get beaten again, train some more, and then win” without losing many major components. These are the elements that define Senran Kagura. It’s not a series that takes itself particularly seriously. Though, there’s thankfully very little of the “have a cliche happen and have someone yell about how cliche it is” tripe that started becoming common around when it first aired.
What’s perhaps more surprising is that Senran Kagura actually manages to wring a decently compelling narrative out of its bare-as-bones components. It’s not an inventive narrative by any means, being a pretty straightforward Good Vs. Evil type deal with only a few minor, well-telegraphed twists, but it’s competently-constructed and fun to watch it unfold. The show has the general sensibility of a popcorn flick, you’re certainly not going to learn anything from Senran Kagura, but it’s a fun way to burn six hours.

Homura, one of the villains, has eight swords. I mention this Doctor McNinja -level combat technique because it's indicative of the show's sensibilities, which is where a good chunk of the fun of it comes from.
With some caveats, of course. Senran Kagura: Ninja Flash! is an ecchi action series, and while complaining about fanservice in a show like this is like complaining about the yelling in Dragonball Z, it is worth, at least, noting down that if you’re uncomfortable with the teenaged characters being mostly-naked a good chunk of the time, you’re not going to get through this one.
Lots of this.Similarly, there are a couple of bizarre-in-a-bad-way writing decisions, though they mostly relate to the characters’ over-the-top tragic backstories and are consequently rather insignificant in the grand scheme of things. The one exception here is the weirdly predatory characterization of Haruka, a manipulative villain and the only textually queer character. It’s dampened somewhat by the absolute mountains of lesbian subtext all up and down the rest of the series (you could probably compellingly pair almost any two given characters), but it does suck a bit of the air out of things.
On the production end; the show is, again, competent but not astounding. While there are some sequences throughout that showcase some pretty impressive animation, it’s mostly functional rather than groundbreaking, and the overall quality of the drawings tends to vary. Most annoyingly, otherwise decent to good fight scenes that take place within the so-called “shinobi barrier” pocket dimensions are hampered by someone’s bright idea to smear the footage in colored filters. Presumably, this is supposed to make them look “otherworldly”. In practice, it mostly just looks ugly.
Sometimes the colored filters strike even in the real world. Such as this one, which appears to have been applied by someone who thinks that it would be neat if the sky looked like piss.One does get the impression throughout that the production may have been shoestring, hurried, or both. Being one of the last notable releases from ambiguously-still-extent studio Artland before they dropped a few Mushi-Shi sequels and then fell down the ecchi rabbit hole entirely before running into serious finance trouble.
There’s not much else to say about Senran Kagura: Ninja Flash! because there’s not much to the series. If you like this kind of thing (and yours truly considers them something of a guilty pleasure), you’ll like it just fine. There is, to be sure, a genuine charm in how utterly goofy it often gets.

On the other hand, if you’re looking for something a bit more substantial, or if the fanservice puts you off, you’re probably better off checking out an at least marginally more restrained “girls who fight” series like Katana Maidens or RELEASE THE SPYCE. If you’re in the former boat but not the latter, Kill la Kill nearly made shows like this one obsolete (artistically if not commercially) just a few years after it aired.
That said, as a final note it’s certainly a likable show despite its evident faults. Girls with swords make for a compelling combination, and while it’s hardly the apex of that particular pairing, it’s enjoyable nonetheless.

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