It has a proprietary protocol and client, though they do use some open source libraries to produce it according to their website (https://discordapp.com/open-source).
The only other thing I can think of is that they have a sizable investment from TenCent which is a China based company that has taken a lot of flak for being involved in censorship within China, see recent uproar about it w.r.t. investment in reddit.com: https://techcrunch.com/2019/02/12/tencent-reddit-nononono/
On the other hand, many companies have investment from them and they do not have a controlling investment in discord either, and there is no censorship for discord as far as I know, but is presumably possible.
Those are the only reasons I can think of not to use discord.