Vampire: The masquerade

Like I said, I'm not completely clear on why it's controversial. I've never even looked at other editions, and have done no more than look at V5 and watch one AP—without participating in any community surrounding either.

That said, the sense I get is that there's a mixture of dislike for aspects of the mechanics and changes to the lore & metaplot. I don't think it's the core mechanics like how the dice pool and hunger work too much (though it very well could be) as it is the mechanics surrounding clans and powers, along with the lore implications thereof.

Edit: oh I should probably also at least mention the Nazi stuff. At least one of the writers in the book turned out to be a neonazi, and some far-right content was snuck into the first printing of the core book. There was at least a perception that the problems were systemic enough to need major change. So Paradox disbanded White Wolf shortly after V5 released and took further development to its own skeleton crew in house plus a lot of reliance on external contractors.