I liked Season 1 back in 2014. It's a dark magical girl show where our cute, innocent heroines are being sacrificed to save the world from extradimensional monster hell. It looks lovable and fluffy, but it really isn't. It often gets compared to Puella Magi Madoka Magica, but for my money it's a warmer show. When it wants to be, it's genuinely fun.
I hadn't expected a second season, but in fact it's a bit complicated. These thirteen episodes comprise:
1-6 = Washio Sumi Chapter, a prequel in which Sumi, Sonoko and Gin get chosen as heroes years before Yuki Yuna came along. If you've seen Season 1, you'll know how many survivors to expect and what physical or mental handicaps they'll have afterwards. (Apparently these six episodes were also compiled into three movies and released into cinemas.)
7 = Hidamari, a condensed recap of Season 1, which is actually worth watching. You revisit the horror, but now from a Washio Sumi point of view. Motivations get complicated. It's an entire season of passion and tragedy in one episode.
8-13 = Hero Chapter, a sequel with all the characters who've survived so far. They relax. They're happy. They think they've won and that the world is safe. Obviously this doesn't even last one episode.
If you're wondering about the right watching order, incidentally, I'd suggest original broadcast order. Season 1 lets its revelations unfold naturally, but you'll be spoiled big-time if you've already watched Washio Sumi. That probably applies even on rewatching, since I don't think you'd want to skip the ep.7 recap or just replace it with a Season 1 rewatch.
More importantly, though, is it good?
1-6: WASHIO SUMI
Most of it's light and charming. It's slice-of-life. The girls are dopey and funny, although the tone always turns serious when a Vertex shows up. (Vertex fights should never be taken for granted in this show, since they're church-sized extradimensional abominations and any one of them on its own could end the world. They're really weird.)
However in more ordinary scenes, the girls will be doing light-hearted stuff like performing for the local kindergarten and following around Gin to see why she's always late for school. They're likeable. Of course we know who's going to die, but the show knows we know and you can watch it delicately putting up death flags. Washio is the goofiest of them, having minimal social skills and having to google "How To Make Friends" while also being a scary right-wing nationalist nutter. No, really. It's played for laughs, but she thinks foreign things are unpatriotic (e.g. Western clothes, Christmas) and ep.3 sees her giving an alarmingly one-sided history lesson about the World War Two aircraft carrier, Zuikaku. Later in that episode she:
(a) salutes a drawing of the Zuikaku.
(b) says, "I'll influence everyone to have national defence ideology through my historical novel."
(c) dresses up as "National Defence Mask".
(d) says she wants to educate the first years in patriotism and turns their kindergarten play into an exercise in militaristic goose-stepping.
(e) gets punished for attempted brainwashing.
In the background, though, we also have the Taisha (English meaning: "Amnesty"). They're trying to save the world. Theoretically you can excuse pretty much anything given that, but I reckon they're a bunch of borderline evil incompetents who are presenting their bad decisions as inevitable and yet will be continually changing their minds or fixing stuff when it goes wrong. The Mankai system was pretty horrible in Season 1, but its first-draft version in Washio Sumi is clearly not fit for purpose. It blows up in your face the first time you try to use it. It's like the difference between a nuclear bomb that gives you slow radiation poisoning and one that simply goes off when you look at it. Alternatively, for another example, see what we learn late in Hero ep.2. They're not surprised! They can explain the situation and they're completely on top of it, but they'd have just left her there anyway!
We also have the horror of them deciding to try the Mankai system in the first place. It's not in place at the beginning. Then they get prompted into thinking Something Must Be Done, whereupon the details of this horrify even the Taisha's representative herself.
This might get a bit much.
This show's format is "nice girls are heading for despair-inducing cosmic doom, but will hopefully be saved in an optimistic last-minute ending." By now we're on the third iteration and I'm not sure I need to see a fourth, fifth, etc. Holy shit, that got grim. There's the usual light-hearted silliness, yes, but Yuki Yuna herself is getting squashed pretty hard and the scenario that's being set up is apocalyptically no-win. Do you trust the Taisha (ahahaha), even though their plan appears to involve freeing the entire human race from the burden of life? It's not clear. This is not a situation where you'd want a lack of clarity. Nonetheless the Taisha think that's the best-case scenario and they might even be correct. (That said, though, they're still the Taisha. One's tempted to challenge them with running a small whelk stall, organising a piss-up in a brewery or perhaps finding their backsides with both hands.)
The ending is pretty out there, but the underlying concept is bold and clear. Watch out for the graveyard of previous heroes, by the way. Yeesh.
Dark Magical Girl shows are so common these days that there's a sliding scale of them. At one end are old shows like Fairy Princess Minky Momo, Nurse Angel Ririka SOS and 1990s Sailor Moon, which weren't afraid of tragedy at all but were still optimistic, pure, good-hearted anime aimed at little girls. They're not deconstructing the genre or anything. Madoka brought in bleak nihilism and despair. Magical Girl Raising Project and Magical Girl Apocalypse are flat-out horror, killing everyone with the maximum of gore, suicide and inappropriate content. Yuki Yuna isn't for children. It was broadcast in the middle of the night. It's capable of killing, crippling, blinding and breaking its heroines. However it also believes in optimism, eventually, and it's easy to care about its charming, plucky heroines. (They're also tough-as-nails superwarriors who can jump like fleas, as does the Incredible Hulk.)
It's a strong show. The Hero arc could be a bit of a slog occasionally, but I'll probably rewatch the whole saga one day.