It's more marketing and appearances than anything else. With Hololive, you have distinct generations and groups that are easily defined by their genmates, (Gen 1 and 2 notwithstanding) and not to mention, they're usually all very unique and strong personalities with equally unique designs on top. Cover may fuck up a lot of things, but the one thing they're good at is talent selection. That isn't to say Nijisanji doesn't have good designs or good talent, it's just that their approach is generally more scattershot and "safe", which means a whole lot of saturation across the board and therefore everything is watered down as a result. Even the bottom half of Holos in viewership still draw more eyes than the middle tier of Nijisanji as a whole. It's also not that wholly unsurprising that NijiEN ended up emulating HoloEN in going for more unconventional designs compared to their newer debuting talents, but ultimately, all of their hires were still "safe" since they were all vtubers in their past incarnations and have interacted with each other multiple times. It's that same kind of reasoning why people can get away with making bait images like this and most people won't doubt it.
For every interesting chuuba like Kuzuha, Claire, Ange, Nui, Lulu, Saku, Mito, or Chigusa, you'll have a share of equally bland ones like Moira, Mugi, Sango, Gar, Cornelia, Sou, Masaru, or Kazaki. Even the weakest Holos usually have some sort of thing that defines them, like ASMR, karaoke, or a specific genre of vidya game, I can't necessarily argue that in good faith for most Niji talents.