^ I am unfamiliar with Junna but, yes, her serious expressions - such as at the start of that 1+1 video - leave an impression 
I watched all(?) of Yuumi's Internet Signature Session (インターネットサイン会) videos (with Runo, Maopin, and Shiori). With the Maopin one she does that "so... you're trying to say you have a crush on me, right?" thing, which made me think of J-Dorama. A little cheesy but we'll see how it plays out.
Re-watching the 2021 TF Budoukan performance, with a focus on Yutti~, I notice that she danced with what I would call excessive sharpness. She does these unnecessary head movements that added up over time would cause dysfunction. As commentators above have said, she has a problem with stamina. Sharp movements are sometimes a way to "get yourself in the game" but doing it too often, relying on adrenaline (dame!), will not end well. The concert was a while ago so hopefully her trainers have managed to change her ways.
If you look at the time 1:41:48 of the Budoukan you will see Yuumi stumbling as she bows, right after Rikoriko gives her tearful thanks-for-supporting-us speech. That is some serious fatigue! 
However, as stated above, that recent 1+1 video is very good 
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