Tbh none of the females seem to be especially evil. Edgy maybe, but not evil.
Lae'zel comes from a obviously harsh society and compared to the other Githyanki you meet, she is OK. She is dedicated to find a cure and to get on with her life without the ticking time bomb in her head, which is understandable imo.
SH gives you approval for saving the Tiefling, being nice to animals and even agreeing to help the Mykonids. Not really evil imo.
Are they likeable? I guess, that depends on what a player like in their companions. I'm OK with both of them (although I would like, if not all of SH dialigues would be behind dice luck), but I want a some more fun loving companion, someone not quite as serious, like Isabela or Sera, as another female companion next.
Lae'zel gets horny for you after you slaughter a bunch of people and she doesn't care if those were goblins or a bunch of refugee kids.
Shadowheart worships a Goddess that wants the living world to end.
I know all of that, I just said, that both don't really DO evil things so far - at least not in my (first) playthrough - and there is so much emphasis in every companion thread, that our current companions are the evil ones, yet I don't really see it? Maybe with Astarion, I forbid him to kill innocents, yet I get the impression, he wouldn't loose sleep over killing someone completely innocent.
I could see Lae'zel betraying us in the future for her people or for getting rid of the tadpole. I don't think she cares, if any of her companions will get rid of theirs, she only sticks with us, because we could help HER.
With SH I have no idea, what will happen. As mentioned, she gives approval when helping the Tieflings and the Mykonids and when you are nice to animals (hell, petting the dog Scratch gives you approval), yet her goddess is not a nice one to say at least.
