Worst TL;DR:

Fandom is a wiki farm, meaning it hosts a bunch of wikis. Also they run on freely available software mediawiki.

Fandom has a couple main problems:

1. Barriers to entry are super low, verification for users takes place 4 days post account creation, with no other steps needed by the user. Paired with the limited options that moderators have for editing access on wikis and you have a wiki that is much tougher to moderate.

2. Ads. Fandom is for-profit. And that means super obtrusive ads that we've come to expect. But fandom also shoved ads in the middle of wiki pages, with admins having no control of where those should be placed. There's also the matter of sketchy ads that are served to minors. Also, some of the ads are outdated but are for subsidiary companies of Fandom.

3. The Grimace Incident. Basically Fandom took over and turned the McDonald's and grimace wikis into huge advertisements, wiping out the hard work that the actual wiki maintainers did. They also put in a bunch of factually incorrect information, literally going against the whole purpose of a wiki and really worrying other wikis, because what's stopping Fandom from getting paid again and repeating the event with their wikis?

I'm sure I glossed over a bunch of the details but that's the best I can do from memory.