I mostly agree with @A_Toad that the extent to which this is jarring really depends on the style. There are some styles where SFS is basically unavoidable, since the faces of characters have been distilled to their absolute most basic shapes while still being recognizable as a human face.
But the closer the creator's style approaches realism, even idealized realism where all the characters are intentionally drawn like supermodels- the more it bothers me. Probably for the same reason jarring anatomy weirdness combined with a more realistic style is bothersome? I think it might be evoking some uncanny valley type of cognitive dissonance. Like if you're not somebody with face blindness or a similar pre-existing condition (in which case I can't speak for that experience of the world)...sitting in a room with a group of people who all have strikingly similar faces that all move in the exact same way and have the exact same mannerisms...would be really creepy.
So I suspect a similar thing happens when you're looking at a comic where the style leans towards realism, but all the characters look and act very samey, like one person wearing different wigs and clothing. Not only does it break immersion...it's a little disturbing on a deeper level.
I've only gotten that feeling from a handful of comics.
I think it tends to happen when there's either 1) A mismatch between the creator's skillset and the level of realism they're aiming for...or 2) It's some kind of externally imposed quirk of the genre that is in fashion at the time, because the creator is imitating the look of a specific actor/actress/model/musician or maybe even another creator's style? (and the "sameness" comes from the fact that it's an imitation, or their dependence on tracing is becoming a crutch, rather than having their own developed style)
It has seemed to me like more of a problem for creators who depend heavily on 3d models of characters, which they may or may not paint over? My knowledge of creating believable 3d characters is basically zero, so I can't speak to that too much...
When it's a more cartoony style with a lack of variety in character design, I feel that's not so much creepy as disappointing. There is so much that can be done to make characters not constrained by realism different from each other that lack of variety there hints at lack of imagination.
