yea, if I was alone in this I might consider myself just biased, but to say "she's extremely nice and amicable as long as you're not a creeper" is just being disingenuous. There's a reason a LOT of people have mentioned how annoying she is. hey, if she's your type, have at it, but I'm calling it as I see it. If I'm so off base, why are they changing her and why is it noticable?
The problem with Shadowheart and the reason why many people find her annoying is a combination of two factors. Firstly, her preferences while sensible from RP perspective are contrarian to how people play RPGs. Shadowheart doesn't want you being too active amd exploring content: she doesn't want you to ask questions and learn about her or the artifact, she doesn't want you talking about tadpole with Ethel, she doesn't want to open the doors to the barn where Ogre and Hobgoblin are having fun, she doesn't want you to open Selunties chest, etc. People usually play RPGs to find challenges and overcome them, Shadowheart wants to evade those conflicts and ignore them, which basically means she wants you to ignore lots of content. Secondly, she is vocal about what she wants and thinks. For example, Laezel only wants to find githyanki creche, but Laezel isn't nagging and whining too much when you do other quests, she only occasionaly reminds you that you are wasting time from her point of view, while Shadowheart is constantly giving advices and directions and is visibly annoyed when you ignore her.
Shadowheart isn't a "tsundere goth emo girl", Lae'zel is. Shadowheart just has an anti-fun personality. You can become best friends very fast, if you playstyle matches her personality, but for many players it doesn't and that's the root of the problem.