Dandwiki has a markedly poor reputation in the online RPG community; material from it is often dismissed out of hand, and users are encouraged to avoid it and to not incorporate work posted there into their games. For what reasons has the site gained this reputation?
There are really two problems:
Zero quality control. Anyone can add anything they want, and since there is no vetting, the overwhelming majority of it is bad. This becomes a vicious circle where the only people who go there are the people who cannot recognize the low quality of the content—which means their own content is likely low quality as well.
Poor indication of what is and is not homebrew. Many things have been put up there in the wrong places and marked as official material when it is not, which makes it very hard to trust it for use as a System Resource Document.
Both problems basically stem from the same source—the lack of sufficient critical mass in editors to maintain the place. Wikis only function well if there are enough editors to mind what the public is putting there, and dandwiki never had that. In fact, it never was clear if people were even allowed or supposed to provide editing on others’ work—which defeats the point of a wiki. This lack of editors also meant there is no one correcting mis-use of various marks of officiality, which leads to the second problem.
The second problem is really the real problem. As they say, 90% of everything is crap, so the first problem is neither surprising nor supremely problematic; it just means you have to spend time and effort finding the few diamonds in the rough that I’m sure are there. But misleading readers about material and making them think it was “official” causes a lot of headaches for DMs and groups and is the real reason that people dislike the site so strongly.
It contains lots of homebrew.
The site hosts pages and pages of homebrew races, classes, feats, spells, etc. Far, far more than the amount of official content on the site. Take a look at the list of class pages.
It's easy, especially for new players, to mistake the homebrew for official content.
Official content on the site is marked with a "SRD" label (example), but the homebrew banner is inexplicably hard to notice despite being bright purple.
When combined with the name (you expect a site called https://www.dandwiki.com to be a wiki for D&D, right?), this means that it's very easy for new players to stumble upon homebrew content, think it's official, and expect to be able to use it in their games. As a DM, saying "no" to this gets annoying after the first few times, especially since...
Lots of the homebrew is super terrible.
There's no oversight on what gets added to the site (it's a wiki, anybody can edit it)! Not only that, but the site doesn't even make it easy to spot the bad entries (comments are restricted to the hidden-by-default "discussion" pages, there's no voting mechanism, etc.)
As a result, the site is full of crap.
The biggest problem with "dandwiki" is that it is named "D&D Wiki" instead of "D&D Homebrew Wiki (that also has pages for publications/products and SRD content)."
I have been a somewhat active user of the site for the past year now and maintain that this is the biggest problem, but most diehard contributors argue that just because it has SRD content and such the title of "homebrew" is inappropriate. This is despite the fact that homebrew is over 90% of what the wiki is used for, and arguably the only reason to use it over better sites that exclusively contain SRD material.
For any remotely experienced DM, the titular inclusion of "Homebrew" alone implies (a) hey this is homebrew, (b) most of this will be crap so use your own judgement, (c) if you aren't the DM, ask your DM about it. Almost every external complaint I've heard about dandwiki originates from three points not being clearly communicated and/or understood.
The "homebrew" banner was globally added to the top of homebrew pages a few months ago to help this miscommunication problem. It's a step forward. I maintain that a title/logo change would do more to help it, though.
Secondary and lesser problems include that despite the extreme efforts of a very small group of people to balance and improve homebrew content, anyone and everyone adds whatever they want, and changes whatever they want—without even needing to create an account. Often these problematic contributors are actively and even wildly opposed to someone else improving "their" work (which coincidentally became the origin story of dnd-wiki a few years back), which only makes the problem worse. Even so, it is at the end of the day homebrew. If everyone who ever visited the site was immediately aware, "this content is mostly homebrew, so use your own judgement," then I am sure the wiki would have a significantly better reputation.
Pages being moved or renamed
In addition to what other posters had mentioned (low-quality and poorly-distinguished homebrew), pages tend to get shuffled around.
About halfway through a campaign that ran for nearly a year, a class I was using (a conversion of Pathfinder's Summoner class) got renamed. Since the class in question had about 7-10 different conversions on the site already, and this particular version had an annoying habit of getting changed (evolution point cost of some abilities was changed seemingly every other time I leveled), it was a pain to find it again. It ended up being sorted way down the page, but I nearly had to scrap my character and remake him using one of the other, quite different conversions of the Summoner class.