I'm not even going to go into the debate about whether darkvision can see through hunger of hadar, the thing that I think makes it overpowered is the this line.
creatures fully within the area are blinded.
That isn't 'they can't see.' Or, 'creatures without Darkvision can't see.' Or 'make a saving throw or be blinded for the duration.' It straight up says creatures fully within the area are blinded, no save, ever. There's no condition under which the blindness goes away, it doesn't say that when the creature leaves the area the condition ends, it doesn't say that when the spell ends the condition ends. The creature is blind.
I assume a lesser restoration could fix this since that ends the blindness condition, but what's the likelihood of a group of baddies having that available to fix all of them suddenly, from going totally and permanently blind? So one third level spell and an entire group of baddies regardless of level have been virtually neutralized. The only chance they have after that is if they have Tremorsense or Blindsight. How is a spell that does this third level?
As written, yes, hunger of Hadar is “seriously overpowered.”
What is written for hunger of Hadar is that “creatures fully within the area are blinded.” Even as an English phrase, “are blinded” is an event, something that has happened to a creature; absent any context, we generally assume that it’s permanent. Consider how common the phrase “temporarily blinded” is, just avoid that assumption.
Now, in context, we have the actual game rules about conditions. Those say
Here we have “A condition lasts until,” that is, until one of the following events take place, the condition lasts, period. Then we have two events listed after “until,” either of which is sufficient to stop the condition from continuing to last:
the condition is “countered,” or
a “duration specified by the effect that imposed the condition” runs out.
But hunger of Hadar doesn’t build in any special circumstances that counter the blindness, so only generic effects that can remove blindness—e.g. lesser restoration—are going to apply. Even dispel magic and the like are somewhat dubious: those can remove hunger of Hadar’s shadows and stop it from blinding anyone else, but it’s not clear that the blindness condition itself is a spell effect that can be dispelled.
And hunger of Hadar also does not specify any duration for the blindness condition. It just says “creatures fully within the area are blinded,” nothing more. Nothing about that blindness ending at any point.
That leaves us with hunger of Hadar permanently blinding every creature that has ever been “fully within the area.” And yes, permanent blindness applied to every creature that ever found itself within hunger of Hadar is seriously overpowered, and that is what is written, technically, in the books.
But we can give more context here—our familiarity and expectations of the game. One of those is that things shouldn’t be “seriously overpowered,” as we just determined hunger of Hadar is, as written. It seems likely that the authors and editors of hunger of Hadar forgot that conditions basically default to permanent, or missed its implications, and considered their wording as “obviously” meaning the condition only applies as long as creatures are within. It seems they almost-certainly meant that the creatures are blinded for a duration equal to however long they stay fully within the area of hunger of Hadar, or that (partially) leaving the area of hunger of Hadar counters the blindness. That would make for a reasonable spell. That is how the majority of people, it seems, assume the spell works, because they assume the spell isn’t “seriously overpowered” and read it within that context without checking the actual technical definitions.
But the game’s authors, really, should have double-checked. They should have included that duration, or that countering event, in the rules text. It wouldn’t have taken much, just a clause like “until they leave the area” in the description. But they didn’t include that. They made a mistake, it seems.
Mistakes happen; there are a lot of interactions in a game system, and ultimately they’re all interpreted by a person who may inject their own sensibilities into things—making it very difficult to recognize that those sensibilities had to be injected in the first place. Ultimately, being careful with that is part of what we pay a game developer for; a large chunk of the value they purport to offer to customers is their expertise and care that allows them to provide more consistent and balanced material than you could do on your own. They didn’t here; that failure counts, even if it is easily corrected. It is fair, I think, to “hold it against them,” to some extent—and if this kind of thing happens a lot, for that to damage your perception of the quality of their product.
But in the end, there will always be mistakes. That’s unavoidable. If you find that 5e is just rife with issues like these, that’s a knock against it. If it happens only rarely, though, then it’s a sign that it’s well-made—don’t over-emphasize any single mistake either. I leave the adjudication on the relative frequency here to others. Hopefully, Wizards of the Coast will explicitly mention at some point that hunger of Hadar only blinds things for as long as they’re within. But in the meantime, their mistake is no reason for you to allow it to mess with your game. You easily can, and should, correct for it yourself.