Well, since no one else took the bait....
The #1 thing these have in common? Family. The villain is either the heroine's father, sister, or son. Which.... isn't where Supergirl stories traditionally go. In most of them she has to direct family other than her cousin Kal-El. Exceptions exist obviously, but, none of the ones I can think of has her family as villains.
Granted, Kara's daughter in Kingdom Come takes an antagonist role in parts of the story, but only in a minor way, and when Superman tells her what to do... she actually does it.
In the New Earth timeline, there's a suggestion that maybe possibly Kara's father had an ulterior motive for sending her to Earth.... maybe. But that's not an active story role, he's a background plot element.
Those aren't people Kara actually even got into a fight with. In fact she's never on the same page as Brainiac's Daughter... her daughter took her place in the story.
So... how would you write a story with a character like that as a threat? Well, Brainiac's Daughter was apparently from the future. Not that we know much more than she existed, but this is an adaptation, not a re-telling anyways. So... what's the worst thing you could do with a half-Coluan, half-Kryptonian? I mean, female Brainiac who can bench-press buildings? Maybe use the spin of she's from a REALLY bad future, and thinks she has to do something SUPER-not-nice to stop the world from ending?
Yes, I know.... that's the plot of the Team Titans book. This is a twist on the Lord Chaos story by design after all. Kara fighting a future daughter of hers to stop something Kara doesn't even know exists? hmmm that has potential. But it's a story they've never done with Kara in the comics. However the point here is looking at what might have been if Kara got used instead of Donna Troy. Brainiac's Daughter is not the same sort of threat as Lord Chaos. But potentially, especially if she starts doing Brainiac stuff, she could be a major threat. Sure, half-Kryptonian in theory means she'd not be able to hit as hard... in theory... but a half-Coluan could probably make a super-suit to compensate. Well probably a lot more than merely "compensating" if we're going there. Immunity to Kryptonite, built in power supply for ensuring she always has yellow sun energy, and more stuff.