but then I realized that she gives more validation consistently throughout the game than the other two female romance interests.
Care to explain what you mean by that?
Probably referring to her post nightsong "I want to be with you now and always" thing or act 3 "I want to share it all with you" thing.
I mean Karlach pretty well does that too the moment you start the scene in act 2. She constantly validates that she wants to remain with you, even knowing she has little time left.
That doesn't cover the lack of communication and scenes throughout act 2, considering by act 2 Shadowheart is very distant and acts like how she'd act if you weren't romancing her. A little difference here and there when romanced would do wonders, imo.