I can't say I've gotten too many, if any accusations of writing a Mary Sue but I have read many of the actual Mary Sues in the fandom I started writing in. They seemed to be going down the list of bad writing errors in order to hit them all. And these were not as parodies but serious, and made for drinking games for how often they stomped their feet and called all other female canon characters little in a dismissive way. Every chapter would manage to make more of the same mistakes as well as find new ones.
I don't use that term in reviews as it is no help if the writer wants to improve, and makes no difference if the writer doesn't want to admit there are any problems. I gave up betaing for the most part, because despite claiming to want help, polite pointing out any errors in setting, period, or spelling/grammar are rejected. Good writers go 'oops, yeah, I can't have loose cannons' in a middle ages fantasy and they make a 5 second fix.
Many sues are in published fiction, Bella and Edward are not the first or last. Paranormal romance has been rife lately, with leads being way more powerful than all the supporting cast. I guess my criteria for suedom includes the purity lists and a more grey short version: does the lead sweat for their win? Not bleed, as violence is now in all genres, but is it hard? Do their fail and have to try again? Do they have to study and do some grunt-work, some boring work that our teenage selves thought we'd never have to face again? Are they just handed the solution, just because they are the special one? Do they have to accept the consequences when they screw around or do things that are mean or stupid? Does anyone who annoys them, end up dead, even if their hands are clean? Is it too easy?
I like competent and good-willed female characters, but they have to actually be good and show it, not tell. If the character is supposed to be kind and generous, they should say nice thigns and help their friends and warn them of dangerous guys. Not expect adoration and to collect their friends' paychecks too. Lobbing a MS accusation without saying why is worthless.
Some reviewers sadly just fling accusations that are not refutable and they won't explain it better. There are so many kinds of Mary Sue, but the accurate times is bad writing, where the Sue is too perfect and spoiled and doesn't have hard to overcome obstacles. Heinlein famously asked something like 'now how can I put their fanny in a bear trap?'
There are enough critical essays and the sue purity tests make it clear that if you remove ALL sue traits, you lead becomes boring and people won't care.
and if the accuser also begs for more of a Mary Sue then it really isn't too bad. Asking for more, totally refutes the accusation, as Mary Sue is a shorthand for bad writing. A good writer can make a Sue into a great story. Many good writers started writing more Sue-ish and improved.
Gary Stues don't get accused as often because whole swaths of Sue traits are 'feminine' traits like appearance and sweetness, which is less often dwelled upon for male characters. Not as many males are written as 'angelic' whose smiles and goodness are an inspiration to even their critics. Male characters have much more agency in stories: they may be strong or clever or really fast with a light saber, but male characters act on the world directly. One Sue, that I kept reading because it was like watching a train wreck, didn't actually do anything, like a pampered toy pet. In a world of magic they didn't do any healing, in many, many chapters, even when friends or innocents were hurt. Males acted, but sue didn't even take the traditional female role of healer and helpmate either, she existed only to be admired. Luke Skywalker in New Hope was pretty Stuish, but Yoda put him into the swamp and made him sweat and he made mistakes => Campbell's Hero's Journey. Sues cannot make that journey or the Heroine's Path if the only flaws the writer admits are trivial, like being too beautiful.
If the reviewers accuse of Sue dom, are they willing to be specific or is this some odd kind of jealousy? Maybe they're inserting themselves into the female OC and they aren't happy that you aren't following the Sue script as they expect. Sues <> OC, I will never say that is an equal set as I write strong female leads who have to work and sweat and suffer for their endings. The traits of a Sue almost always include a love interest, and are well described on wikipedia and the tropes on tv site.