I think the hardest thing to do is having complex facial expressions overlapping when characters talk. You could do face capture for every dialog option but that would be a massive task.

In alot of engines characters mouths are controlled by a lip sync system that uses, pitch, tone or text fed dialog to ‘mimic’ words being formed in the mouth. It’s far easier to have that and then having facial expressions as a separate animation layer that’s blended together and triggered based on a enum that’s selected by a script (say a players dialog option says “Your a mean man” and the player selects it, the NPC knows what you selected and in that dialogue option theirs a little enum (it makes more sense if you treat a dialogue option as an object) that contains the facial expression or expressions that are appropriate to use in response).

Full facial animations are used mostly for cutscenes because actors cost money while in game is just the engine trying to move the mouth using code (I know Farcry 5 had this where only the important characters had full facial animations and the rest just flapped their mouths up and down).