I think the big difference between hololive and nijisanji is their designs.
JK Gumi is cool and all, but they're high school girls. Its impossible to feel attracted to anything under the age of 18.

Meanwhile hololive designs are ambiguous, they could be any age from 18-34 and you can work with it. This means that hololive designs can be sexualized while its literally impossible for the jk gumi and many of the other nijisanjis which come off as "loli"