The biggest problem is that federation splits the communities between instances. So a single interest may have 5-10 different communities, all with fewer users than on an equivalent subreddit.
I’ve been saying this since the beginning, Lemmy needs a way to follow topics that allow you to subscribe to all related communities at once. And posting to a topic on one community allows it to show up across different communities of the same topic.
Something racist about the way black people smell, I think is what they mean.