Gen 0 (Sora, Suisei, Robocco, Miko)
Sora and Robocco has comparable lower viewer numbers but they are always part of representatives as far as promos and big concerts outside of holofes. Their SC is quite good too. Like top 50 worldwide last 2020.
Gen 1 (Aki, Mel. Haachama, Matsuri)
They are all struggling except for Haachama as far as viewer numbers but as far as SC. They are in the Top 20 except for Mel. But still, they are very profitable
Mel paused her activities for a time and was demonetized or banned quite often.
Gen 2 (Choco, Subaru. Ayame, Shion, Aqua)
Choco was the only one struggling except when she would do collabs with other holos. She's pretty much on the same boat as Mel except the stalker issue
Gamers (Fubuki, Mio, Okayu, Korone)
Mio is the weakest as far as SC and viewership but her Collabs streams and participations are one the of the most entertaining content I have seen in Holo. HoloKindergarten, Food Report, Haiku competition, Okayu's Trial, Aqua's MC hiring spree, even the HoloFes. So no, there is no weak link.
Gen 3 (Flare, Noel, Marine, Pekora, Rushia)
Flare struggled but her MC streams pulls 10k viewers consistently and all of the Gen 3 are in the top 10-20 as well for SC. Their merch solds out in minutes.
Even if Noel does ASMR just like Mel and Choco, it is not the only thing she does and not really why people watch her.
Gen 4 (Coco, Watame, Towa, Luna, Kanata)
I would say only Towa and Luna are struggling but they have one of the most loyal fanbase so I won't say that. They have built a community around them. I wouldn't say that any are struggling.
Gen 5 (Botan, Lamy, Polka, Nene)
For a time it was Nene but she has gained new viewership so no weak points here.
HoloEn (Ame, Calli, Kiara, Gura, Ina)
Comparing numbers, some will get more views that the others but the other person would earn more SC so no weak links.
HoloID - I can't really say. I don't pay attention to them except for Ollie.
TLDR - It's only the members that were focused on ASMR before or were promoting themselves with that in mind are the one that are flailing somewhat. Choco has switched to more MC streams along with ASMR. Ditched Ark too. Mel is doing more collabs too.
It's not the number. It's the type of content they dish out.