Put me down for not being a fan of the system in BGIII.
Look at Lae'zel as an example of why this is detrimental. Lae'zel is about as close to a 'blank slate' among the Origin characters as we get. She doesn't have an epic backstory, ambitions, relationships etc that sets her apart from Tav-Gith. She's basically as 'vanilla' as you can get as far as Githyanki in her position as you can get. SHe has a predetermined personality, but again- that's not really too distinguishable from what you might expect from any 'normal' gith in her position with her background. So in theory, whether you are playing her or as a custom Gith, there shouldn't really be any differences in the reactivity of npcs, your dialogue options, etc. Yet there seems to be a lot of content you just won't be able to experience as a TavGith that you get as Lae'zel. That goes into dataminined stuff, but it seems that Shadowheart for instance, will not have the same relationship with the player that she does with Lae'zel if you play one or the other, you don't get options to do any of the 'crazy Gith stuff' that Lae'zel gets to do, like pulling a knife on her companions that first night.
Basically, the TavGith experience already seems like it's set up to be a stripped-down experience of the Lae'zel experience, even though just with a few tweaks like pronoun changes Lae'zel's origin is pretty much set up already to be a great experience for custom characters.
That's the sort of 'cost' to having this Origin system we are seeing-not even getting into other stuff like the lower number of companions and the 'main character syndrome' most of the party members have with their elaborate, 'epic' backstories.
