As someone who romanced Karlach very often, Shadowheart lacks sorely in the 'I love you' department. It's like both her and Tav are scared to say those magic words somehow.
I get it, actions can show love but it's also nice on occasion, to tell your LI you love them or hear it from them.
Totally agree with your post.
She calls Tav "true love" or "love", but no option directly tell her those words do suck.