Immersion.

To me implies coherence with the storic period used for the fantasy set.

D&D revolves around the european middle age. I've studied those centuries, indeed they were as far from the "dark ages" mythos the enlightenment build up in their will to make the Renaissance and spreading humanism the breaking point of a new age of wonder and development and discoveries.

Nevertheless those were years ridden with violence, intollerance (for instance there were uncounted massacres brought in name of religious believes between the growing catolic cristhians and other interpretations of christianism), one culture watched the other as wrong or inferior and used divine right as a justification to conquer, pillage, subdue other populations, some ethnics were persecuted all over [other than the prejudices against the Jews in this period started the prejudices against the semi nomadic populations of Roma and Sinti], women have to fiercely fight to have a role that was different that of faithful wives that take care of their husbands and offspring, etc etc.. ...].

Indeed is due to the work of monks that cultural works from the classic roman and greek period were able to survive the turmoils that came after the fall of the empire and the raise of the roman - germanic kingdoms, in this years are implanted the seeds thet will brought the rise of the concept of modern nations.

All of this is lost in D&D.

Specially with the aftermath of these years rightfull battles of minorities (specially the black one) that has pushed to avoid some aspects that could be controversial.

That comes at expence (for me) of immersion and credibility.

Let's take the thieflings: they are peculiar and unique, they are the offspring of demons, not followers of an evil or chaotic deity. Demons, as much as I can try I can not reach the point were I can depict any demon being something more than beings that follow a clear objective that is to corrupt other sentient beings leading them to committ non moral or ethic acts, and withe ultimate goal to gather a number of souls enough to allow to win the ultimate battle against the good deities.

No matter wich culture demons (or creatures alike to them) haven't the access to the "complexity of gray shades", they are evil, bringer of chaos and mayhem.

The thieflings are the spring of the union between humans and demons, pretty sure the demonic part should play a non forgettable role. Obviously the part that they are in part humans allows to create a playable character, but it would be more difficult to just push aside the demonic component.

In a medieval setting concepts like those we rely on that are based on biology don't apply, specially if mystical entities are involved.

Furthermore a more smart approach would make use of the discrimination and intollerance rooted in a medieval setting to show the difficulties of persecuted minorities (for characters that belong to a due race, Drows follow an evil deity that has marked them as her own, a deity that pushes them to be deceitful, manipulators, a Drow that doesn't want to follow that heritage has to fight their way and change deity, also they have to gain the trust from the other races; a Thiefling is a demonic offspring again if they want to get rid of that part of their identity they have to fought hard because I'm pretty sure a demon won't take so easily that their offspring does what they want, on the other side they have to relate with the prejudice of the other races), and how easily is for who doesn't belong to a minority to fall in prejudices and to follow intollerance.

Kagha is a good example of wrong management of said chances, she just look like a racist extremist, had they done the dialogues, and gave her a different look, it would have been a more striking way to show how racism rooted in our (all, racism and intollerance are not a monopoly of european derived cultures) societies, for example I remember how my mother (a very lovely and somehow altruistic woman) described my will to try not to fall in prejudice (I say will to try because I continuosly analize my thoughts to understand if there are uncoscious bias) as "you know he has a kind heart", I was appalled because differently from my father (that is openly racist, something that is quite ironic as I was adopted from South America) it was more nuanced, there was no aggression just patronizing and belittlement all said with the most lovely and cute tone of voice.

How much frightening would have been if she had the aspect of someone we associate with kindness (like Aunt Ethel), and talked about the good will and heart of those that didn't share they decision, and how she was forced because she had to think about the lives and wellness of her people, or that Halsin has lost his focus because of unbeliavable curiosity and need to do the right thing thus putting himself, and the lair, in danger?

Instead we have a npc that polarizes the players, and whose actions in the end
are belittled because she is a dark druid
.

Furthermore there's the stark contrast with how the Goblins are transversely despised [the only reason Halsin was againt a full slaughter of them is because he is a druid and so he preserves the balance].

On the last point of the original post: the mindflier in the underdark has a solid background, he is a fugitive, an heretic on the run, he has mastered a way to hide himself from the elder brain and isolate from the mindfliers's hivemind, and he has ti live in one of the most dangerous places in Faerun that is the Underdark, furthermore the player's party has to gain the trust of his friend before being able to meet him, and lastly he stats that he is more unique than rare, moreover in the dialogue options there are those that show suspicion and lack of trust on the heretic gaith.