Contractual Purity: Sally Amaki (who portrays Sakura) has stated during a live stream that she and the other girls are expected to uphold the image of a "pure" idol, and none of the members were allowed to have social media until May 2018. Even then, they are only allowed to follow their group members. None of the members are allowed to date, aren't allowed to say anything that might shatter the illusion, and to limit public posts to talking about work and 22/7 (i.e. no Fangirling) but Sally has managed to get around this by speaking English to express things that she's not quite allowed to say, since she relies on the fact that her management doesn't know English. And let's just say that she's pretty lucky that her management and the majority of her Japanese fans don't understand English. (She did, however, have to tone down her behavior after being reprimanded several times by her management.)
In her interview with The Japan Times, she admitted that the management actually can understand her tweets but they try to respect each other's boundaries. Also, she sometimes posts pictures of her anime and idol merch collection and talks about her biases in Japanese.
Defictionalization: The group's concept is "idols that transcend dimensions", where voice actresses portray their characters both in animation and in real life.
Ensemble Dark Horse: Sakura Fujima became wildly popular in the Western fanbase when her actress, Sally Amaki, revealed herself to be not just a native English speaker, but also a regular purveyor of Internet memes, Idol Singer culture (both Japanese Pop Music and Korean Pop Music), anime, and fan fiction. Her One of Us nature resonated with a lot of Western fans, which endeared her to them. Out of all the members, she has the most Twitter followers and had at least 7,500 followers within the first week of creating her official account (whereas the other members had less than 1,000 around the same time).
Face of the Band: Miu (played by Nagomi) is in center position in all choreographed dances and the music videos put a lot of focus on her.
Nightmare Fuel: Episode 6 ends with nearly everyone fainted due to food poisoning off from coffee jelly the staffs bought for them. Even more terrifying when Reika, Sakura and Kono passed out and their bodies were littered in front of Jun who was the only left standing.
Tear Jerker: Episode 7. While the main plot of Jun filling in for all the other girls is lighthearted and funny, her backstory certainly isn't. She was born with a respiratory illness and was sent to the hospital frequently as a child. During her stay in the hospital, she met another girl named Yuu, who helped her open up and become more positive, and the two became very close. Some time passes, and Jun's condition improves thanks to Yuu's influence on her. One day, she's happily walking to Yuu's room and opens the door...only to find that Yuu has just died from her illness. Poor Jun runs to the rooftop and bawls her eyes out, crying that she should have been the one to die because of how negative she had been, feeling that Yuu deserved to live more than her. Jun makes a full recovery from her illness, and feels as though Yuu gave Jun her life to her, and resolves to be a happier person.Damn.
The Wall also reveals to the girls that they were just the first step in its plan to change society permanently. Exactly what that plan was, or what it entailed, was never revealed.
Values Dissonance: Episode 6 is a major point of contention among Western fans of the show. The girls have to do a photo shoot in their swimsuits, and Reika is adamantly opposed to doing it because she doesn't like the idea of showing so much skin and reducing herself to some sex object for fans to ogle over. Everyone else, including the girls and the manager, urge her to do the photo shoot anyway, to the point where some actually point out that she's not being serious about her job and she's burdening the rest of the group. Putting aside how rigid and exploitative the Idol Singer industry can be, from a Japanese perspective, Reika is seen as selfish for holding the group back from their job, as they can't complete the photo shoot without her. From a Western point of view, Reika's concerns would be viewed as perfectly valid, as she's a teenager being expected to let herself, an underage girl, be sexually exploited at the expense of her comfort, dignity, and personal boundaries. Many Western fans really don't like this episode, to the point where one person wrote a Fix Fic centered on said episode.
Wangst: Miu's monologuing in the first three episodes of the anime quickly go from the lack of self-confidence of a shy girl to just plain whining. It's even more noticeable compared to Jun's Character Focus episode, where the main moral is explicitly that Wangst serves no purpose because life is too precious to waste it moping.