i haven't really listened to a lot of kpop, but i disagree with that as well. They're decent singers and entertainers.
The only thing is now, they don't have is a real lead singer. I'll have to say that none of their voices has the lead ability. Maybe Seungyeon who has great vocals, but there are times when i can't distinguish her from a lower register Nicole.
You know when Gyuri is singing, but its not the belting powerful type.
Nicole can potentially have a good high voice, but she's really pitchy when singing live, which makes me believe they use a lot of editing/melodyne or whatever on post production studio work.
Jiyoung has a voice similar to Emi Hinouchi. Its great, soft but with a tinge of raspyness. Not a lead though.
And Goo Hara...well... she looks like Namie =P j/k
She has the weakest voice of the bunch, but it could be worse. She's starting to get more parts, and is getting more consistent, so thats good. But like in Lupin, her lines were "its mine". In Honey, she had 2 lines.
Since Sunghee left, they lost that power voice. But whoever their producer is, they have arranged their strengths and covered their weaknesses very nicely. Jiyoung and Hara usually take the first line or 2 of the verse, then have Gyuri and Seungyeon switch the 'lead' to finish off strong. Then when things get settled in, they put in Nicole here and there to keep the energy going and have the track dynamic and refreshing. Its a great formula and compliment each other very well.
Whats also good is they each have different voices and you can tell who is who (besides the previous fault i said). Another good group is Brown Eyed Girls. SNSD, they all sound the same to me... soulless! =P j/k i have to listen more to them (sorry snsd fans =P)
For the current Kara, i'd rank Seungyeon the best, then a tie with Gyuri and Nicole (on her best day), then Jiyoung, then Hara.