I'm trying to figure out why having 'flawed' companions in the sense that they all have a dark haunted past makes it better for story telling or RPing. How does that work exactly? Why is having untrustworthy companions better than trustworthy companions?
Well i said in general because it's a simple way to identify with them or to set something to strive to change movign forward, but it's not mandatory and some become dull and repetitive too.
But, what makes them flawed is not their "dark haunted" past though, is the reasons why they end up having those issues or why they remain tangled in them:
-Gale was a prodigy in magic, he had an amazing future ahead of him, yet his desires transformed him in a ticking bomb; a proudful wizard became someone that almost has to beg to you to give him magic items, and it's a bit trapped because even killing himself will probably make him explode and kill many more. His flaw is falling prey to his desires, his curiosity and ambition.
-Shadowheart is on an important mission, she doesn't know why, yet she knows that is important; she knows that is for Shar and she is devoted to her, yet there are points in which she doesn't understands the purpose behind the needs of her godess. Her flaw is her blind faith, one of his traits is that she lacks individuality and she often seems to fake her opinions based on what a Sharran would do/think, when you ask about her she gives you really surface level responses or extremely deep a subjective ones because she doesn't have anything in between.
She "accepted" to have her memory erased, she is no more than a tool, and it's interesting to me because her story wil probably be her accepting this or rebelling. It wouldn't be crazy to me if she changes faith and subclass at some point in the story based on our actions.
-Lae'Zel is someone forced to work with cultures she often belittled. She acts proudful and secure, but there are a situations where she shows her insecurities, in her bluntness she tries to hide the lack of depth she has navigating this new cultures. The game shows it clearly, she would die if not for the simpathy of the main character, most of our companions and the tiefling will kill her because she threatens them even when in peril.
Her flaw is her lack of empathy, her lack of awareness, trying too hard fulfill a destiny and a role between her society that bears no importance in this new world she in inhabiting and even among her peers.
I don't want to make this longer that it needs to, I'm with you that some of their stories are a bit TOO big to my taste, but those are not their flaws per se.
