Kara only works as a Superman supporting cast character. Her origin is too close to Superman's to justify her own book, which is why none of her ongoings have ever really worked. As a supporting character, the idea of "Superman's older cousin who is also younger, who is more Kryptonian but Clark forces her to acclimate to life on Earth" is kind of interesting. And yes, I know the TV show's a big hit but Kara on there is just "female Superman," so TV casuals don't actually care whether she's her own character or not.
Matrix (the pocket universe protoplasmic superpowered shapeshifting clone of Lana Lang created by Lex Luthor to be sent to Earth-1 to recruit Superman to fight the Phantom Zone villains who then later dated Lex Luthor II and then merged her body with a dying Satanist named Linda Danvers) is obviously insanely convoluted but at least PAD took a crack at streamlining her origin/powers/personality that was actually starting to gel (after 60 or so mediocre issues) when she got tossed out for nu-Kara. His idea of turning the book into "Supergirls" with all 3 (at the time) current Supergirls in the style of Birds of Prey was a neat one.
Also I like the idea of the Super-family being more people that Clark took under his wing rather than strictly his family, in the same way that Bruce does.
tl;dr Matrix's origin sucks but she's at least unique