Unrequited love...such a sad thing
Shadowheart has a personality that suits her mechanics in the game. Both horrible. She's far the weakest companion and Larian should at least re-stat her to make her relevant (ie. str/dex swap that would fit her thematically). But keep at it, behind those thorns there is apparently a rose waiting to be discovered!
She can have the highest AC of all characters if I recall correctly with spirit armour and mirror image, plus she can charm and guide people making skillchecks much more likely. The decoy spell works pretty nice, too. The mass sneak spell is also pretty good in certain situations. She is decent in melee, too, and isn't healing word only a bonus action? So you can heal and pick a party member up at the same time. But yeah +1 dex and +1 con would not hurt. She can still dish out some ranged damage with her two ranged spells with a decent hit chance, when you have height advantage.
She is rest intensive, though, I'll give you that, if you do not rest a lot she will be dead weight.
