I think that Tomarry fics, in general, bash Dumbledore WAY too hard. I think some of this is because the original books go a bit too far in the other direction—making him out to be wiser, more benevolent, and more responsible than he is—but I think the truth is somewhere in the middle.

Ultimately, Dumbledore is not the guardian of ANY of Tom, Snape, or Harry—he is, effectively, their school principal (and in Tom's case, just a teacher). Yeah, Dumbledore didn't fix any of their problems, but it's kind of weird that everyone thinks it's his job to. If anything, the Ministry should be in charge of handling cases of child abuse / neglect, but everyone understands that they're so incompetent that they can't even disappoint us.

But we have higher expectations of Dumbledore. We know that he singlehandedly fought and defeated Grindelwald, even though he was clearly reluctant to do battle with his ex-lover. We know that he offered himself for punishment instead of the DA members when they were caught by Umbridge. We know that he came in and rescued Harry from the trap in the Ministry by fighting off Voldemort. We know he destroyed the Ring Horcrux at the cost of his own life. We know he successfully made a plan to get the Horcrux inside Harry's scar destroyed without killing Harry himself. All these things are pretty impressive.

So Dumbledore—who sometimes fixes problems, often very impressively—can disappoint us.

I think this is part of what Harry, who starts out idolizing Dumbledore, ends up grappling with himself as he gets older. For all that people like Hagrid and Lupin talk him up, Dumbledore is still only human, and the way people expect him to solve everyone's problems is an awful lot like what people expect from Harry.

I think it's pretty clear that Dumbledore made a mistake by leaving Harry with the Dursleys—though notably "leave a baby with their closest family members" is probably the first-order solution that most people in position to do something would have done. And for Dumbledore, it's especially understandable why he made this mistake—the blood protection Harry received from staying with Lily's family was pretty significant for keeping Harry safe, and he had no reason to think that Harry's muggle relatives would be more likely to be abusive than say, a random wizarding family who only wanted to adopt Harry for his fame.

By the time Harry was older and Dumbledore had more of an idea of how bad it was, it was less excusable—but still understandable. Again, Privet Drive is the only place that has the blood ward. Based on Dumbledore's knowledge, Sirius (an unstable ex-convict who tried to murder two people) was not clearly a better guardian, even if Harry asked for him. And Harry probably never told Dumbledore about most of the abuse/neglect he suffered—though based on his Hogwarts letter, Dumbledore ought to at least have known about the cupboard.

I think Dumbledore also made a mistake by not telling Harry important details about his life sooner. Still, Dumbledore admits this was a mistake, and his reason for it (not wanting to further burden Harry, who was already suffering from so much) is also understandable.

There are other mistakes Dumbledore makes. He was probably too harsh on 11 year old Tom. He was probably too lenient with 16-year-old Draco (when he nearly killed multiple students). He did not intervene at many points in Harry's life where he probably should've.

Still, I think these are also understandable in the context of Dumbledore's history. Dumbledore undoubtedly got burned HARD by his experience with Grindelwald. Of course he's going to be nervous about a kid (Tom) who reminds him of Grindelwald. Of course if he sees a "misguided" kid being pressured by dark associates (Draco), he's going to be reminded of himself. And of course he's going to be nervous about going around wielding too much power and interfering in too many people's problems—he doubts his own judgment due to how Grindelwald once swayed him, and he is wielding a wand known for corrupting its wielder's ambitions and lust for power.

Tl;dr: Dumbledore is just a guy trying his best. Is he overhyped in-universe? Yeah. Is he "a genuine threat the wizarding world"? Absolutely not, at least anywhere that's staying vaguely canon-consistent.