If you use a custom PC I see no gameplay difference between Origin characters and companions of comparable games. The only difference is story-related, before we were settling family matters (Anomen, Keldorn), face an adversary/trauma from the past (Jaheira, Aloth), an extra dungeon quest (Korgan, Valygar) or some/all of the above (Eder).
Now we have companions with such extreme issues that could potenitally shake the foundations of the eart itself. You could argue that the theft of Boo is also such a world-defining trauma, however each of the Origin story is overwhelming and simply not relatable any more, since you do that 3-4 times over.

If you yourself chose to be an Origin character, I suppose you get that for yourself, but still have to face equal issues of your companions, that also share your tadpole, so the innate drive of the main quest. So the main quest is actually not an individual issue, but a group effort, which I find a really fresh idea tendencially, but it clashes with the equally important personal quests and a general lack of group interaction (and interaction in general). The bonus is you get a pre-defined character to play as, with a backstory you do not have to make up for yourself. That surely is an advantage with many people and a popular choice. I bet many people never changed the appearance of Commander Shepard for the exact reason that they wanted to play HIS story, not their own.

However, as we are playing BG3, it clashes with the predecessors, where the whole point was to experience the Bhaalspawn's story. In BG3, compared to the Origin characters the PC is just an accident; the equivalent of the possessed bakers and fishermen you see on the illithid ship. I find that annoying.