It's a bit of an exagguration to say that osu! has the cringeworthiest community to date, but that's just my opinion. Take a look at some YouTuber's communities and some maybe CoD.
Good question, though. I myself can't come up with an answer that can be considered worth reading as of now, so I won't try to tackle the entire question. I do have a few things to say about #osu though:
I think everyone can agree that #osu is a complete and utter shitstorm, this is probably partially due to the lack of moderation at certain parts of the day, and the types of people who generally use #osu. As for my daily experiences in #osu, they usually consist of questioning why people are so attached to my username and why people won't stop spamming the most uninteresting of memes. That in itself is a pretty unexplained answer, I'd be surprised if anyone can actually come up with an in-depth answer to the entire question, and not just why #osu is so cringeworthy.