The faces themselves are important, but basic structure is always more important than individual features. Look at someone you find attractive, chances are they have a slightly crooked nose, their ears are small, their teeth aren't all straight, etc. Human faces have a vast range, but there are anthropometric human measurements that almost all human faces share regardless of race or age (obviously a baby's face does not line up with an adult's face) that are important to incorporate when building a character's face for any form of art. Put eyes way too close together, or way too far apart, make a nose, mouth or even ears too low or high and it ends up looking wrong to the point that it triggers an unpleasant feeling in the back of your mind. These measurements don't have to be rigid, some people's noses sit lower than others, some people have bigger ears, but when you go well outside the average, and do so with multiple parts of a face things start to look awful really fast. I don't think all characters should look like the "ideal human", but there is a point where things start to look really off.
As much as structure is important, it's the quality of facial animations that really make a game's characters stand out. A perfect face model can look like shit if the animation isn't up to the same standard, pulling the believable face model back into uncanny valley. Fallout 4 is a great example of this. The faces in the game were extremely detailed and considering the range of the character creator they looked better than I ever expected. You could add any number of imperfections to faces, and the faces wrinkled and morphed in a very realistic manner. They were so well done in fact I never replaced the vanilla faces in the game with a modded version despite the massive number of mods I was running. Unfortunately for the game, the sheer number of voiced lines (100,000+ lines) meant motion capture wasn't an option and as a result all that effort was wasted on what looked like puppets being manipulated when characters were talking as the facial animations weren't even retouched.
Another extremely important aspect of a character model is the eyes. When you talk to someone you spend most of the time either looking directly at their eyes, or looking away completely when you become aware of the fact you've been looking directly at their eyes and don't want to seem weird. If a character has dead eyes no matter how good everything else looks, the character look off and again creep into the uncanny valley. Kevin Spacey in Call of Duty is a perfect example of this. The model was about as perfect as it could be, they used motion capture for the scenes, but the eyes were at best unnatural and I couldn't not see the empty dead eyes of the character.
I'm finding that despite the fact games are advancing at a technical level, with more polygons and better shaders, often times the art or facial capture team isn't up to the job and the faces in more modern games fail to look as believable as many older games. Over-exaggerated animations often take what would otherwise look like a perfectly acceptable model and make it look completely unrealistic. Halo Infinite's pilot character is a great recent example of overdone facial animation, where the same character in the 2nd trailer for the game was more reigned-in, but there have been many titles over the last gen that have fallen short as well. Andromeda is a good go-to for critique as it was absolute dogshit. The Original Mass Effect had some pretty ground-breaking facial animations in it's cutscenes, but from there Bioware seemed to have hit a wall. By the time Mass Effect 3 launched the facial animations were actually worse than in the first game as they appeared more wooden, with transitions between expressions being almost mechanical. Andromeda was a clear case of a lack of the proper talent, while the rest of the series decline was a case of a lack of effort that not surprisingly coincided with Bioware's slide into obscurity after the EA acquisition.
Enslaved: Journey to the West has some of the best looking facial animations of it's time. Playing the game years later they've really held up well. It's not like the faces are jammed with modern effects, it's that the facial expressions are animated in a manner that doesn't look overdone at all.
Ninja Theory pulled out all the stops for facial animation in a game and used Andy Serkis for facial capturing and relied on his input, then the team went through the animations and retouched them to better fit the models. Andy is not a mountain of a man like Monkey and his facial structure isn't a perfect match, so things needed to be adjusted for everything to look right. The results were believable character expressions and memorable faces in a game that even for it's time didn't push any graphical boundaries. Despite this, the game's cutscenes never felt like they were stuck in the uncanny valley even considering the fantastical dystopian future setting of the game.
At every step in the game, Trip and Monkey show a wide range of expressions that always look sincere. When trip realizes her entire village has been killed and comes across her father's body, she doesn't grit her teeth like a maniac, open her mouth like she was about to eat a really big sandwich or squint super hard like she was about to shit out yesterday's really big sandwich. She genuinely looked lost. It reminds me of actual people I've seen during terrible moments in their lives, and the emotion the facial expressions added to the game made the game better than the sum of it's parts. The same could be said for Monkey, he spent 90% of the game being really, really angry, but his facial expressions mirrored my own when I've felt the same anger. I don't scowl the entire time I'm pissed off, but my face shows my anger through subtle cues like the brain aneurism veins that pop on my temples and the lines that form on my forehead. Again nothing was exaggerated and the character came across as believably angry.
Trip
Monkey