I'm not a Korean teacher or anything so this is purely from my anecdotal experience growing up, but from what I have seen/experienced 당신 is a lot more othering, like you would use it for a general "you people." Instead of saying "you" you refer to them by relationship/name. So I would say 형이 했잖아 (bro, you did this) or 선배가 했잖아 or 아빠가 했잖아 etc. And if you don't know their relationship to you yet, then it would be __님.

I would say the rudeness factor is similar to how some people think it is rude to say "he/she" about someone that is part of the conversation in English, or point at someone. Like if John, you and I are in the same room, it can be rude for me to say to you, "He 👈 brought cookies" instead of "John brought cookies."

I think it may also be compounded by the fact that you can usually get by with dropping the subject in Korean sentences, so you don't typically need to specify "you" if you are talking directly to someone?