Chaeyoung mogged everyone except jihyo during 16 that's why she was chosen. I think she just stopped trying after debut because she realized she already made it for being in the big 3, similar to the aespa girls, she's still very cute though.
Dahyun was chosen because JYP wanted a non problematic christian girl. She is okay as an idol but she doesn't stands out at all, she was made to be one of those cute filler members all girl groups with a lot of members have. I like her post chorus in Talk that talk, I'm happy she's getting more vocal lines and less "rap" lines lately.
Momo doesn't need to know how to sing when she's already known as the best main dancer of the third generation.
I don't think Twice has any useless members except maybe Sana but Sana is still an asset due to koreans liking her visuals and the fanbase she brings to the group.