I've found myself totally enamored with this show. It does a few things so right that it lets me overlook a lot of the "bad" parts to the extent that I've started to think "well, maybe it's actually fine the way it is?". I think if you, like me, love these parts then the show will mean a lot to you as well:

1. As everyone has mentioned, the relationship between Sumire and Kiyo is lovely. It's occasionally over-acted for my western tastes, but the idea that you can have a friend who supports you as deeply as they do each other is a fantasy for many of us. Personally I've always struggled with the codependency that most romantic relationships entail, but Koreeda has depicted a truly wonderful alternative. Sumire and Kiyo are as close to partners as you can be platonically, but the communal living involved in 舞妓さんち forces them to interact and support those around them as well (after all, the makanai's job is to provide food for everyone). Communal living is a hot topic these days as loneliness and isolation increase dramatically (at least in America). Koreeda paints an *almost* utopian version of it.
2. "Almost" because we find that not everyone is happy despite the warm and cozy look we have into their lives. People find themselves feeling homesick or even losing motivation and quitting. But the whole thing is done in Koreeda's understated way. Like in real life, we never really understand how others feel. We only get a glimpse into why this fantasy world is insufficient for some and have to fill in the rest ourselves. To me, this is so much more satisfying and *real* than the vast majority of in-your-face jdrama scripts.
Watching Sumire deal with her "love triangle" in a quiet and subtle way felt all too familiar to me. One of my favorite moments--finding out from Sumire that Kenta was awestruck by a Geiko when they visited Kyoto on a school trip and the implication that perhaps that's at least part of the reason she became so interested in it. Something that I would miss in real life, presented the exact same in this drama. And, finally, the episode detailing her relationship with her father was beautiful. We don't know exactly what's going on in her homelife. It doesn't feel like abuse nor total negligence. But just enough disconnection so as to feel relatable to any teenager (though in this case they actually manage to move away at 16!), while simultaneously still holding onto the desire to seek one's parent's approval.
3. Finally, if you like learning about Japanese culture, Makanai is rich. It's clear that one of their goals was to teach us about maiko/geiko culture. We're naturally exposed to how difficult it is to sleep without ruining their made-up hair, to life without cellphones as a maiko, and to how apprentices are basically personal attendants to their mentor geikos. It's not entirely clear to me that these are all "real" but they at least felt real and believable to me (and, FWIW, I was introduced to the show by a native japanese who knows a lot about this world, so it's probably at least not wildly inaccurate?)

To that last point, I'll admit these flaws exist but also cover why they don't bother me very much:
1. For a lot of people, the reality that both alcohol and super old men with super young women/girls being core to the work is inherently gross. I do think bad stuff happens. But my impression is that it is at least not "normal". I think it's okay for this to not be a full/balanced documentary as long as I'm aware that it is perhaps, one-sided. The same could be said for almost any wholesome drama, I imagine.
2. I didn't love Nana Mori's (Kiyo) performance here, nor Makita Aju's (Ryoko). I understand Kiyo is supposed to not be as physically capable as the others, but a couple episodes in I was wondering if she had like a mental and/or physical disability. It's *so* exaggerated at points (and then, thankfully, disappears at other points). I think if she just toned things down it would've been so much more powerful (props, again, to Natsuki Deguchi (Sumire) for doing exactly that). The script here makes up for it though--the small scenes she has with everyone (esp. with Momoko and, separately, with Tsurukoma) end up being powerful just because they portray so perfectly what a moment of listening and empathy can look like even with people we aren't that close with.
As other's have mentioned, Ryoko's story seems to not have a satisfying ending. In general, I had a hard time sympathizing with her despite her having a legit reason to be angsty (i.e. growing up without a dad). I wonder if this is a lost in translation thing, but she honestly felt more like a weirdo than a bully (... why is she eavesdropping in sitting in random places all the time?). And when she sort of repairs things with her mom it just felt... fine? Good? I'm glad for her, I guess. There was no particular reason for me to care about her/nothing likeable about her IMO. I feel like a few scenes to showcase her hobbies/interests would've been enough. Though, on second viewing, we do get a glimpse into stuff like when she talks about her latest "bias" (i.e. kpop idol crush) with a couple characters, so perhaps my feelings will change here.

Lastly, while there are a lot of threads that are loose (e.g. what happens with Yoshino and her husband? Why didn't they want Yoshino in the house, anyways? What about Kenta/Kiyo/Sumire? etc) I think I almost like the show that much more for it. Obviously I'd love a second season, but I see the show as a glimpse into their lives. A glimpse which feels all the more authentic when we don't see the things leading up to their leaving Aomori nor the resolution to the stories we've become deeply invested in. Or, as they say in the show, an 一期一会.