That there is no nuance, and a strict set of guidelines (i.e. this is the safe ship, the safe method of shipping, the safe au) within the larger groups of antis makes it easier for predators to get into those groups. The insistence that everyone who engages in X content is Safe but those who engage in Y content are Bad makes it distressingly easy for someone who wants to prey on younger people to appear Safe and Acceptable, even if they are older, by many years, than the kids and teens they're engaging with.
No, you dont have to think abusive ships are okay, or that eroguro is fine, but you have to be aware that there is more to people than the surface layer of shipping and thematic preferances in their fandom engagement.
What about an adult fan creating "questionable" content, who tags their work appropriately, doesn't intentionally interact with minors, and keeps to themselves and doesn't engage in those themes in general discussions is More Troubling than an adult who goes out of their way to ingratiate themselves with groups of minors in the fandom, engaging ONLY or MOSTLY in the content MINORS are putting out, consistently initiating and maintaining contact with them until they're considered "close" friends?
Minors should absolutely have a safe space within fandom, and there absolutely is content that shouldnt be brought up or discussed in general within the fandom. But the way to go about keeping spaces safe is utilizing the tools at our disposal i.e. tagging systems, blacklists, and filters to keep content we don't want to see away from us.
Predators know how to infiltrate groups and take advantage of situations. Being vigilant is key, recognizing manipulative actions is key. Black & white labeling of ppl as Safe or Not Safe based on something as arbitrary and easily faked as their shipping preferances and thematic preferances? Not key. It may give you insight, but it shouldn't be the sole reason you decide someone is Safe enough to interact with. Look at their actions as well.
Its like the whole "If you want to know a person truly see how they treat the waitstaff" or whatever, there's more to people than their ships and fandom tropes.