What are the privacy concerns for using Discord?

While I don't have the entire picture, I can say a couple of things that have turned me off of the app as a whole. You have pretty much no control over your data, and you must supply a telephone number in order to setup an account. Many developers create communities on it though, so it makes it hard not to use. Personally? Non-data related - I find it insultingly busy. I am not the target audience though.