to be honest, i find this very hard to articulate in a manner which won't easily be taken out of context, but if i had to explain, they write geto like he's a heroine in a y/a novel or like how they think a woman should act, and also as if he's had etiquette training classes as a young lady since he was young? i don't know if i'm explaining too well, but they write him very submissive, how they think traditional "feminine" people are, and demurely. people seem to forget geto was as much of a meance as gojo is, but they always write him as if he's the chagrined wife looking at her troublemaking husband, instead of his partner in crime that would regularly get into shenanigans with gojo....he's also very one dimensionally written, a lot of the times he doesn't retain any of his canon personality traits, and satoru is also written like your stereotypical dom alpha male. they make him overly callous, uncaring of geto's feelings, and often make it like gojo treats him more as a sex object than an actual person. geto also seems to be much more sensitive of the difference between him and gojo, not even in a power/strength way, but literally that gojo is from a wealthy clan and he gets anxious about it, even to the point of crying about it sometimes, when we've never seen evidence that this was smth that bothered either of them in canon. geto is a very strong-willed character, with strong convictions and personality in general, but they make him seem as if he's very frail and emotionally distressed all the time. he would not stand for people disrespecting him, we've never seen him have a problem standing up to authority (even tho he will act respectful by default), and he's someone that was born proud. you don't come into an entirely new world, with a class that has the strongest sorcerer in the past several hundred years, and are able to stand up to them without a very strong sense of inner strength and confidence. yet, in goge fics he is very insecure and anxious, and will often need gojo to "take him with a steady hand", so geto can cling onto him and gojo can "take care of everything" from what geto wears to how he acts. but in canon he's perfectly fine casting aside anything he's ever known to embark on a path he believes has meaning, he's never had an issue carving out his own path which we see from his introduction to his death, and is very unlikely to ever want or need someone to have that much control over his actions. in fact, he'd probably find that uncomfortable and annoying, much like gojo would because they're equal in strength of character.

they also will take away any agency geto has in his actions, like anything bad he does will always be downplayed or they'll push the blame onto other people/circumstances. any morally grey decisions gojo does will be amplified so it's completely negative instead. i've discussed this with some of my goge/switch friends that later switched to gego or sukugo, and they've all also said that the manner in which gojo treats geto in these fics is very forceful, you can kinda feel a slight discomfort a lot of the time while reading them. all these things are noticeable enough that i can tell when a fic is written by someone that leans goge or a goge author even if it's not tagged with top or bottom dynamics. i know not ALL goge fics are like this of course, but for the people that scream about canon characterization, if you ask jjk twt, most of them will agree that there's issues in the way goges characterize stsg.

of course, i should add that gegos aren't perfect at this either, we can also make stsg ooc sometimes, but it's never to the point i'm thinking "wow. who even are these people? these aren't even the same characters?"